You definitely do not want to succumb and retaliate. There is no end game there, except that users leave both forums as they are spammy and unusable. There, a

You definitely do not want to succumb and retaliate. There is no end game there, except that users leave both forums as they are spammy and unusable. There, all you've succeeded in doing is killing off two forums. First, contact the admins and establish communication. Explain what happened, and ask if they would mind changing their ToS or put a notice on their board, asking their members to stop. From this round you can figure out if the admins did it, or if their users did it. If it was the users, then you should probably look to technological solutions like CAPTCHA, user bans, IP bans, etc. Maybe require that someone be a member for X months, or have N minimum karma before they can PM someone else. Start filtering for certain content (URLs, names of other forums, etc.) If it was the admins, you should plan on doing the same stuff, but you're probably going to be up against some more savvy folks. Start watching your logs seriously, and step up the tech filters. You can do things like flag suspect accounts, and filter messages to / from those accounts (if you do it silently, the other site admin won't realize his spammy techniques are ineffective). Step 1: open lines of communication Step 3: profit!