Dude, if every time I joined a BG and everyone focused on me just because of who I was, I would /afk as well. EVERY SINGLE TIME. You guys are essentially griefing him and then laughing/complaining when he refuses to play along.
Step back and take a look at what you're doing, then think about how you would react in the same situation.
We used to let him play,but since it was well known he'd do this, we just focus him for the initial battle. He's gone after that. And it usually happens in AP games we do this, which isn't that often because there are alot more games going on that hell get into.