Is there a way to have preset discussion threads in a forum? (no student created threads)

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I would like to have a discussion forum with 3 preset threads that students can choose to reply to, and then there would be ongoing discussions within those three threads.

 

The problem I have right now is that students open the forum, and then instead of replying to my preset threads, they are creating their own threads, which segments the discussions and makes it hard for me to review all the ongoing conversations.

 

I have tried my best to explain this process to students, but they just see the big blue "start new thread" button and can't resist clicking on it!

 

So is it possible to prevent students from creating new threads in a forum, while still allowing them to reply to my preset threads? Or am I thinking about forums and threads wrong? Thank you in advance for your advice!