I'm terrible. I taunt them if they message me first with a negative message.

Like,
"Why do you cheat?"
"Because I'm better then everyone else."
"Your a loser."
"Thanks."
"Duel me."
I ignore duels, requests for duels, or anything as soon as I get into questing. It takes too much concentration.
"You won't duel me because you are a failure at games and life. You are a loser."
"First, I was kidding about being better then everyone else. Second, I wouldn't consider collect 100 armaments cheating. I enjoy multiboxing because its a challenge. And I would lose at dueling because my interface is set up to do instances and kill people who attack me, not to win duels."
"Oh."

"Is it worth it?"
"Yes."
"Do you really pay $90 a month?"
"I spend more on a night out in town."
"Yeah right. You have no friends."
"...says the 76 Death Knight?"

Sorry MB community. I just love to argue to much. It's a fun challenge that gets me thinking about the most ludicrous way to respond.
I ignore WTF tells. I don't attack anyone unless they've attacked me first in the past. I'm always willing to help.

The most annoying thing is people who go, "Invite." when I'm already fighting an elite spawn that they want. I already have 5 people in my party, and I wouldn't have time to type /invite whoever while fighting. Those people usually attack the elite twice then leave. Funny thing is though, if they would've stayed around and helped me kill it, I probably would've returned the favor.

I figure all in all, just keep a list and you'll know who not to group with in the future when you solo. If they are so immature already just imagine how horrible it must be to group with them. The answer I'd like to say to everyone who messages me with a negative tell to start would be, "I do it so I don't have to group with spoiled brats like you. This is just more fun."