It's a judgment call in every case. The primary philosophy: Establish, maintain, and protect an atmosphere of mutual respect.
The clearest indicator of a person not doing that is vandalism. The next, page blanking. Not much to decide, in those cases.
Grayer cases: Refusal to discuss an edit. Honestly discuss, that is. Stonewalling, abusive and rude rejoinders, constantly picking fights, racial slurs ... all the usual trolling techniques ... simplify the decision. These can be summarized as deciding "Is the person a pain in the ass to be around, and is it worth the hassle?"
Sometimes it is. Solid contributors sometimes can have Single-Issue Wonk
or Hot Button
issues we can work around because of those other, more solid contributions. Regrettably, sometimes the answer is "No, not worth it."
Being hostile to new people is a pain in the ass. Talking
to a newby and trying to orient them toward having a friendly discussion is not a pain in the ass. Being a newby who won't or can't get it: ass-pain.
That's the outline.
Edited to add: On durations: A ban usually doesn't happen unless it pretty plain the person is intransigent. We have lifted one or two, to see what happens, and been bitten. Still, it's a judgment call. Depends on the situation.
edited 15th Nov '09 2:12:55 PM by Janitor