I think it's better to not allow probation to post threads till they become members . it will be better as the spammers will register alot of time if they wanna Spam
I mean this in the nicest possible way, but you must be a spammer to have thought of that :-).

It is a very good point though. However, restricting the number of threads they can start to say, 1 or 2, would already be a deterrent to 'spam' as the time it takes to register E-Mail addresses and so forth may not be worth the hassle. That being said, probation members have to type in a captcha to post so perhaps they are willing to 'invest' that time.

What may be a prudent course of action would be to restrict the posting of threads to 1 a day, and see how it goes. If it doesn't stop them, ban them until they become full members!