I've been banned by HN twice, no appeal, no explanation, in fact you don't even know because you can still log in and post comments. The first time it took me a couple of weeks of "how come no-one replies to my posts any more" before I thought anything was wrong.
The first one - I have no idea why - I guess the -4 (which I still think is a valid comment)
http://news.ycombinator.com/threads?id=skwiddor - 100+ karma
Ok so next time round I'll be more careful, got 400+ karma
Then one day you slip in a joke and you're toast