I wasn't particularly called any names for losing; rather, I was told I suck because I won.
My very first Smash U FG match was where that happened. I wasn't playing extremely well... I am far from a pro. I was just playing well enough to destroy my opponent, that's all.
So after the first match, he renamed his tag to "USUK." I hadn't considered it being anything but an alternate name. So I beat his patheticness again. This time, he renamed himself, "USTILLSUK."
...Ah. So THAT'S your game. Once I realized what he was trying to say, I rematched him one last time just to kick his *** out of spite.
I seriously hate people like that... But, even though I was insulted, I just didn't take it that seriously. I mean yes, it was annoying for like five minutes, but then I just laughed it off. If being told I "SUK" because I won is supposed to be taken seriously, I'd hate to see how I'm supposed to react to a close friend or family member dying.
There's absolutely no way you can force people to stop acting like two-year-olds, especially online. Realizing this, I just like to let things go nowadays (unless I'm at my limit).