You can lock a forum when you want to prevent users from contributing to the forum or making updates to existing posts. If you do not want to lock an entire forum, you can control access at topic level by locking forum topics.
You must be the Forums administrator or the forum owner to lock a stand-alone forum or a topic in a stand-alone forum. To lock a community forum or community forum topic, you must be a community owner or the Forums administrator.
Parent topic: Managing forums
Working with forum topics