When the lifecycle of a forum topic has reached its natural conclusion or if instructors wish to prevent students from making any more posts in it, forum topics may be locked. This can be triggered manually on specific topics or set to occur after a certain period of inactivity. Note that it is also possible to set a cut-off date for forum posts at the whole forum level (which would be common when a forum was being used as an assessment.) 

To manually lock a specific discussion topic in a forum:

• Select Lock this discussion from the 3 dots icon at the far right of the row for the topic on the forum list view:
  
• Select Lock this discussion from the 'Settings' link at the top right within the topic itself:
  

To set forum discussion topics to automatically close after a certain period of inactivity, access 'Discussion locking' in the forum settings and specify an inactivity duration:

Please note that: Forums that are locked manually using the first two methods above; can be unlocked. However, Forums locked due to inactivity cannot be re-opened.