Right now we don't have a system (automatic or otherwise) in place that bans a user after they reach a certain negative karma. Usually a poster who is generally annoying to the community ends up breaking a guildline or two causing me to discuss these things in PM with them and usually that puts them on the right track. Also other members will write to be privately complaining about them, will cause me to take action (depending on the problem) and speak to them privately about their behavior being disruptive to others. I'd say 99.9% of the time a PM clears things up.
Generally it's a case by case basis, but I suppose that if we got someone that hit -100 chances are they'd be pushed out the door soon enough as to not cause further disruption of the community. I'd imagine it wouldn't get that far though; if they were being that awful, they'd likely be banned before we could withstand 100 negatives. *grin* However on Karma alone I would not ban someone. If they had that many negatives, I'm sure the reasoning would be easy to see though. One is the side effect of the other.
Does Karma have any meaning in a banning? I have no doubt if I saw the person had -20 and they had recently done something that was unforgivable that I would have no hesitation in banning them, verses a +20. But even then I'd still ban them for the unforgivable action even if they had +20. Karma does let me know how others feel about a posters contributions to the community in general - which is helpful in these things.
The karma would never be the ultimate deciding factor - the persons behavior would ultimately ban them. This can mean someone could be banned that has positive or negative karma. Karma would be too easily abused if power or weight was added to it. It is certainly good to know how the community feels about someone though when I ban them, as I don't enjoy banning people. I don't usually ban without really good reason. Poor spelling has never gotten a ban, nor would it ever I can't imagine ^.^
I highly encourage you and everyone to write to me if they have problems with anyone at all. If rules are not broken I will still act on complaints from the community because I wish everyone to feel comfortable being here.
Whoo that was long winded. I hope you got the answer from it you were seeking