This is for mages only:
1. At the start of any BG when people are getting ready, start opening up a portal to anywhere... exodar is the best.
2. While you are casting the portal, type out "CLICK THE HEALTHSTONE PORTAL PLEASE". Press enter to send the chat juuuust before the portal is made.
3. Watch the humor as people start dissapearing and you see "Player has left the battlegound."
Note: these people recieve deserter debuffs and can not play another bg for 10 minutes lol
-I have had people create level 1 alts on my server and curse me out after I did this too them.
Another one for warlocks in AV:
1. Summon people on offense to the cave with the screeching sirens.
2. Summon people off the bridge to their deaths.
Of course you would lose the battle if you summoned the whole offense to the siren cave, but it would sure piss people off.