How about removing links from the forums for a few days?
Adbots spam for Google search position, so they don't care if the link is invalid. Besides, this will also be annoying because legitimate links will also be invalid. I therefore say no to this idea.
Search the Net and you easaly find out, that some (not all) bots can deal with CAPTCHA.
The security of a CAPTCHA is determined by how hard it is for a computer/bot to separate the characters.
What i found when i was playing one game to the next i found the same comment on each one. The comment said
This game is great but i know a better one
then a url takeing to another site contains a game that is i quess better? i dont know but i think it was spam so i flagged it. i was suprised to find out no one else had.