Most negative behavior does not result in an immediate ban; that is reserved for things like threats and hate speech.
However, any behavior that is creating a negative environment for the other players may eventually result in punishment, and if you continue that behavior the consecutive punishments will be increasingly harsh and can move up to a permanent ban through this 4-tier system.
The rule of thumb is just don't be a jerk. If it is probably going to result in bothering the other players, consider not saying it. It doesn't matter if you think they are thin-skinned pansies, just leave them alone and focus on winning the match instead of making it worse for the other players and lowering your chances of that.
You can find guidelines for sportsmanlike behavior in The Summoner's Code and the Behavior Worthy of a Champion article. If you are uncertain if a specific behavior is negative or not, feel free to ask here and I'll do my best to clear that up for you.