I've only known of two students being kicked out, and both for the same reasons.
Student a's case was a lack hygiene and lack of respect for other peoples air. At the age of 16, he left a solid turd on the dojang floor, completely throwing an atom-bomb in my instructor's lesson plan for that evening. Before and after the butt-biscuit disaster, he had no qualms about flatulating in the dojang. He did it often, silently, and noxiously. His doboks tended to get washed only once a month, and I'm guessing you can imagine the green aura which surrounded him. Eventually, my instructor realized enough was enough, and wouldn't let him resign after his contract let up.
The second's case was simply his flatulence. He would constantly let out terrible odors that had the freakish habit of having his own distinct stink. Behavior modification was tried, where he would get 10 push ups for every person who smelled it (our advanced classes have upwards of 10-20 people). This didn't work, and he was basically told to go to another school.
Attitude tends not to be a problem, as the ones who don't shape up usually end up quitting due to intense physical discipline. Most, however, do shape up for self-preservations sake.
I understand the first one... the second one, it would depend on why the flatulence continued; I had a student who had a digestive disorder and he could not control the problem - we kept air freshener around otherwise ignored it. If it's something controllable and the student chooses not to control it, that's a different issue.