1. Asking questions like that one.
2. Not slipping us the monthly hush money.
3. Turning radioactive.
... do you mean historically, here, in terms of permabans?
Off the top of my head:
2. Trolling/Disrupting the forum
3. Posting porn
Most people respond to the occasional infraction by changing their forum posting style, so usually there is some concerted effort to be disruptive that results in a permaban.