1. On the Anchor_GoBack_GoBack navbarthe navbar, click Discussions.
2. From the context menu of the forum or topic you want to lock, click Edit.
3. On the Restrictions tab, select your Locking Options.
4. Click Save.

Watch how to lock a discussion forum

https://www.youtube.com/watch?v=4eVXGFttrJE&feature=emb_title