I tend not to delete comments (except for obvious spam) or ban commenters. If you post more than one link, I will rescue your comment out of the Junk Folder once I discover it there, no matter how much I may personally dislike what you say.
I let Creationists' comments stay - nice fodder for my regular commenters to debunk.
I let Serbian and/or Albanian nationalists' comments stay as long as they do not cross the line of proper behavior (e.g., physical threats).
It is the last few weeks of the election season so I am posting a lot of posts on politics. The emotions are high, I understand. The comments by folks defending the GOP (or collecting McCain brownie points) will remain, as long as they do not cross the line.
Who decides what "crossing the line" means? Me, of course. This is my blog. If you would not say something in my living room, in front of my kids, don't say it here. We can be adamantly in disagreement yet remain polite. If I decide you are too crude and your contributions useless, and I delete or ban you, there is no Court of Appeals - this is, after all, my blog and I can be as capricious as I want to be.
My Mom reads this blog, sometimes my wife, my brother, my kids, my friends and neighbors. I have a reputation for having a friendly blog - keeping the comment threads clean is part of it.
So, when someone like Mr_G, on a day when he forgets to take his meds, finally discovers where his computer's "On" switch is and decides to be a jerk on the internets, his ass gets banned. Get out of my living room. Go back to AOL boards. This is a family and science blog.