Todd Bailey
IBM Technical Support
The following document lists preferences and suggestions to help prevent users from deleting calendar entries. It is recommend to enable the following features and educate your users about these calendar features.
1: Create a policy to set safer default values for C&S related preferences.
Create a Mail Policy to remove the Remove / Delete dialog as an option to users who live in their Inbox / All docs folder. On the Mail / Basics tab, the "When user deletes a calendar document from any view or folder" setting should be changed to Remove without asking. This setting will cause any manual delete actions as a Hide request if it is done from any non-Calendar view. If a user really wants to delete a meeting they can still select and delete it from any Calendar view. Doing so will also cause a Decline workflow message to be sent out (or a Cancel in cases where the user is the meeting owner). The "When user deletes any document in the sent view:" setting should also be set to Remove without asking. This setting has the same effect as the other one when the action is taken from the Sent view in particular. If they still wish to delete the document, they can do so from other views or folders.
Note: If this setting allows users to select Remove or Delete, please be sure to inform all users that hitting the delete button will delete the meeting forever. If this is performed while in a mail view such as the All Documents view, no calendar workflow will be engaged. Meaning if a chair accidentally deletes the meeting from the All Documents view, no cancellation notice will be sent to the invitees. This feature is documented in TN# 1292578.

In all of the policy settings above, the settings are best applied as Set value and prevent changes although Set initial value would also be acceptable.
On the Calendar and To Do / Notices tab, the "Display the following meeting notices in user's Inbox:" setting should be set to either All except responses or All. On the same tab, the "Remove meeting notices from user's Inbox after user has processed them" setting should be checked. This will still allow users who live in their Inbox to participate in C&S workflow but it make them use the Calendar views to remove entries that they have already processed making it less likely they will accidentally delete them without generating workflow.
Since some users prefer to live in their All Documents view, it would be a good idea to check the "Don't display calendar documents in the All Documents mail view:" setting on the Calendar and To Do / Notices tab. That will prevent those users who could otherwise delete a meeting from the All Documents view from having that ability. If they want to delete it, they can delete it form any Calendar view just like the setting above. The same could happen from the Sent folder so the "Don't display meeting invitations in the Sent mail view:" setting should also be checked on the same tab.

The final preference that may be set to safeguard end users is the "Ask me before deleting Calendar and To Do documents" option. This will simply display a dialog box to users warning them that the meeting cannot be recovered.

This option may also be set with a policy.

The policy can be both applied to existing users as well as used during user registration:

Note: Policies are a push model. So if a policy is set to "set value and prevent changes," to unlock it in the future, we recommend that the policy should not be deleted. That'll leave the last value set (but unlocked) for the end user. Instead we recommend unlocking the setting first and setting the value to whatever the default is or what the previous value was. That will in effect restore the user experience to what it was before the locked down setting change was made.
2: Educate end users on good mobile device usage
Mobile devices make it easy and convenient to stay connected to work. They are also a potential source for problems when it comes to C&S because not all of them fully understand the Notes C&S model so they sometimes can do the wrong thing. To reduce the chance that a mobile device issue will cause a problem with C&S, users should only use them to view their Calendar contents or create new ones. Making changes to an entry and accepting/declining invitations from the devices have been found to be sources for problems at customer sites in some situations (especially with repeating meetings).
The problems that arise often become exacerbated when multiple users are involved such as the user and their AA. This is because there is an increased likelihood that potentially conflicting changes are made by each user in different locations. When this happens it becomes very hard to untangle the mess and decide what the final state of the calendar entry should be. To further complicate things, sometimes 3rd party software does not handle the confusing data it sees well and can wind up causing more damage thus further complicating the problem.
Some mobile device clients like Traveler have built in safeguards to avoid doing the wrong thing but not all of them do and the safeguards that exist may not catch every possible scenario. To avoid user confusion the simplest suggestion is to simply use mobile devices as "read only" devices for Calendar contents. Users can create new entries on their devices but should avoid making changes to them from the device once they are created since the changes may or may not be properly applied in Notes.
It is also important to know that users may install personal synchronization software on there devices or on a personal computer or laptop. These applications are used to synch Notes email/calendar documents to personal calendars which are often used to access Notes calendaring information on a calendar such as Google, Yahoo, or Hotmail just to name a few. There are several applications that may be downloaded for free so it is extremely important to discourage this type of behavior. These types of applications have been known to delete or negatively alter meetings in a Notes mail file. If it is a must that a user use such an application, we strongly recommend that the application is configured to perform one-way synch from Notes to the third party calendar. This should prevent bad information from being sent back to the Notes calendar.
3. Calendar Cleanup
Often times, users simply delete calendar entries in an attempt to reduce their mail file size. This holds especially true for users who may be restricted by mail quotas. To prevent this behavior, please educate your users on the calendar cleanup feature which is found under the More menu. This option deletes old calendar entries based on two main criteria, the $NoPurge value and the Modified date listed in the document properties. This will delete old calendar entries without deleting repeating meetings that still exist or any other calendar document that is needed for a current or future meeting. This feature simply runs archiving for the calendar, but the documents are actually deleted.

4. Duplicate Repair Tool
With a mix of admin assistants, mobile use, and multiple mail file replicas, there is an increased chance that a user may experience issues with replication conflicts. These documents may cause duplicates to appear in the calendar, and often times, users, or there admin assistants, will attempt to repair these meetings themselves by deleting the replication conflict document. However, this may cause additional damage to the meeting that cannot be repaired. To resolve these issues, the user should open the main document, and they will be prompted to repair the meeting. All they will need to do is follow the prompts to fix the meeting. More information about the Repair Tool may be found in TN# 1326680.
Related documentation:
Notes/Domino Best Practices: Calendaring & Scheduling