It's easy to tell when I've got IM spam from a robot. I always have a quote after my name in MSN, and the robots, trying to be personal and therefore more accepted, tend to say things like "Hey, mokalus - 'Like Summer camp for drama nerds.', check out http://[redacted]". No human would bother contacting me out of the blue to give me a link and copy what is clearly a quote rather than part of my name. The robots fail this basic comprehension and reveal their true nature.
Mokalus of Borg
PS - Technically, I suppose this is a form of CAPTCHA.
PPS - It was not intended to be.