Some questions and problems with user editing permissions

[mathforum]

I run a math forum and many times we get people who post problems and after they get help they delete their original post in order to cover their tracks. Sometimes people quote the original question and it's ok but sometimes not.

So my question is, is it possible to limit editing to only the first post? And if so, can I put a time restriction on this as well? If this isn't possible, is there a way to document or a current way where I can view all changes made to posts and store all text entered into the database even after it's changed?

Thank you for your time.