Locking a topic prevents users from creating new posts or modifying existing posts, making the conversation read only. Users with permission (Instructors and TA roles with editing permissions) can modify the contents of a locked topic. Locking a topic is an effective way of closing a conversation while allowing users to refer back to posts. Consider locking a topic to end a conversation, then using a pinned summary post inside the topic. When users access the topic, your summary appears at the top.
You can specify start and end dates for when a forum or topic unlocks so you can set things up ahead of time to match your course calendar.