Good question. There really is not anything that will get you banned without warning, and any game ruining action can lead to a ban if warnings are not heeded.
Things that I feel are game ruining, and can lead to a ban:
Repeatedly spawn killing (camping) in any game with the exception of protect the zep, and then only in the last few minutes.
Any spawn killing done with a tank.
Team killing, the auto ban deals with this, but if I see someone team killing who has multiple complaints by multiple people posted on blacklist, then I may ban.
Things that I feel are game ruining, and can lead to a mute:
Excessive profanity. It is not such a big deal for the occasional "oops". But when it gets out of hand, and a warning does not work....
Personal attacks in chat-box on an individual.
I will not ban for any action that a mute can stop. I leave the forum for the other mods and zup. Most of my muting is done in the world chat.
I am very reluctant to ban. I will " kick" rather than ban all but the worst repeat offenders. I will not hunt players down to ban them. I review the black list two or three times a week and take action on those who have a lot of complaints made by a lot of different players.
As for bomb droppers: I think that most of the time it's poor sportsmanship, but not always. Those on the short end of a 2:6 game have good reason to drop. I do not take action against droppers, if it is bothering me, I just leave.
I know that there are mixed feelings on moderation. Some feel mods take to much action, others feel that not enough action is taken. Some want longer bans/mutes. As a player, forum member, and global moderator, If I could do one thing to improve moderation it would be to get more global moderators. 15 or 20 more responsible moderators who only take action in game would go a long way toward reducing the occurrences of game ruining behaviors.