Well, What if we made it so they don't post for a month, and then the ones who actually care would send emails to be evaluated. If there are thousands of people who join, most of them look for awnsers, in a month they might get an awnser and go away, this would reduce a whole lot of users sending emails to the admin. However, even then there would be alot of emails, and it would take someone with dedication to want to wade through those emails.
Maybe just make it so they can't post for a month and at the end they do a forum quiz. If they cheat with google to find the awnsers at least they will learn enough to not ask questions that are awnsered in the FAQ.