Interesting question, CarlZ.
This is the same issue we face here, and it basically takes a lot of work to moderate the site and keep things humming along nicely.
To aid in the process, I guess you need to make the interface as clear and usable as possible, to give visitors the best chance of using the site appropriately.
we dont allow users to post more than once in a thread.
That sounds odd to me. Surely the point of a thread is to be able to ask/answer questions, clarify points etc. What's the thinking behind that decision?