Really, it becomes a major annoyance.
Maybe require a valid, previously registered, product serial number before allowing people to register on the forums? or whatever, but please remove the spam and porn from these boards.
Because I was 'slightly' pissed at all the spam I inserted keywords like spam, fraud, spyware etc in some of those spam threads, at least they'll get their 'products' and links associated with those keywords on search engines.