GOG.com: We have added another preventative measure to battle the many spambots that have been attacking our forums - it is no longer possible to create a forum thread if you only have the set of free games you get upon registration in your game library (adding posts to existing topics is not affected). This should rid us entirely of unwanted spam threads, as spambots will be unable and/or unwilling to purchase games.
If this will prove to be a big inconvenience for users, we will remove it - but it should rid us entirely of spam. We will continue working on solutions less restrictive to other users once the flood of spambots is under control.
My mitigation suggestion would be to add a line of text somewhere in the account sign-up process indicating that until the new user has bought a new game (or acquired a new game, if freebies that they "buy" count too), they can't create any threads. Of course, some people don't read, so a sticky thread in each sub-forum would be useful too. And maybe a sticky thread for potential buyers to ask questions if they can't create a thread.
Yeah, this solution has problems, but it can work as a stop-gap.