So every time you're in a game with him, quit out and log another character, or AFK in the BG and play another F2P account, and start playing war games*
Every time you see the biggest douchebag in WoW, make it very clear what you think of him, in every way possible, without risking a ban, copious use of a /spit macro is recommended.
Unless most of the people in the BG give him a very clear 'We're not going to play with you, so f*** off' message, he'll keep playing (and if you don't tell him to f-off if he's on your side, why should the other side tell multiboxers to f-off if they start playing against you).
I used to multibox 5 accounts before I stopped paying for WoW, but rolling hunters of all things in a low level bracket is just low.
*Can we get a serious effort to get everyone's HS set to Shattrath, so we can get everyone into war games regardless of their faction, even if it's only 2v2 arenas.