Prevent original poster from deleting forum thread once a reply has been received

I've noticed that some posters wait for an acceptable reply and then delete the entire thread - possibly because it was homework and they don't want the answer to be traceable. I don't think this should be allowed for 3 reasons: 1. It would discourage people from asking homework questions in the first place. 2. The answers given may be helpful to other people. 3. If you need to refer to an answer which you've previously given when replying to a new thread, you know it will still be there!