Sucks about the concert, wish things could have gone better for you. Hopefully he'll get the picture you're not interested and leave you alone.

As for 4chan, IGNORE IT! /cgl/ is nothing but spreading lies, rumors, and just making vendetta attacks for no reason. I've had a lot of friends who cosplay, one in particular who gets posted on there a lot because she's a well-known California cosplay and she just laughs off most anything 'anonymous' tries to say against her. Hell, even i've been mocked on there before... It's best to just ignore anything they say.