Why not just send them a message letting them know you have to go, but would like to get some matches in another time when you can stick around longer?
I definitely agree with people taking it way to seriously sometimes, some of the anger levels I've seen generated from online matches is ridiculous. Though the rage mail can be pretty funny at times, it would be nice if people would at least attempt to use punctuation and stop misspelling lose. (so many people online are loosing to bs spam and other cheap tactics)
I don't like when I've been waiting in a full lobby for a match and someone who just lost and must have revenge messages me demanding that I pass, doesn't happen that often but the few times it has have been irritating.
Also uncommon but mildly annoying is running into people that demand or beg for you to let them win so they can get a trophy or achievement.
I can't agree with Tsubaki being a pet-peeve, she is awesome! (I will concede that her hat is kind of dumb though)