I've never booted someone from a game. I have, however, not invited a regular gaming friend to a game and not told him about it. He found out and was very upset, and won't spend time with me anymore to this day.
We didn't invite him because he likes to create unmotivated, hard-to-motivate anti-heroes, and it makes for rough gaming. When I GM, I don't care, as I'm used to handling it... but the GM for this particular game was my girlfriend at the time (now, my wonderful wife) who was running her first game ever, and was really nervous about it.
He's the kind of guy who gets upset at the drop of a hat, so part of me knows that he would have acted this way if I hadn't been subversive about it and had simply restricted his character to one that would go along with things, so either way I was screwed... but for a long time I couldn't help feeling bad about it.
However, it's now been years and he still acts this way. I refuse to apologize anymore and just ignore it. I made a mistake, but I apologized profusely. If he can't accept that, it's his problem. He's the one who doesn't game anymore because of it.
As for quitting games, that can be hard or not depending on the group. One group I left currently (who still thinks I'm on hiatus) was hard to leave, but I just didn't have enough time, and if any group had to go, it was them. But it was difficult to leave, especially considering that a week prior to my no longer attending, they had suggested switching my Cthulhu game to something else because they weren't really into it. It was hard to quit outright at that point, as I didn't want them to think they had offended me (which they hadn't... Cthulhu isn't for everyone, and they really like happy, high-powered games like Nobilis and Changeling), but I really wanted and needed to quit.
Also, one of the guys in the group doesn't shower or wear deodorant much anymore because his hippy girlfriend has convinced him not to... that also acted as a catalyst to my leaving the group. He smelled really bad, and the odor would linger sometimes.
So, there you have it.