Like most online games, Blizzard has the power to suspend or ban people from WOW, and they're not shy about using it. Half of the Customer Service Forum is full of people complaining about their banninations. Bannations? Bans. Of course, no one in the CSF has ever, ever done anything wrong with their account, and tend to go to ludicrous extremes to deny they've ever cursed, shared accounts, or gone AFK in the battlegrounds.
The simple 3-hour or 3-day suspension is probably the most common form of ban. So you swore at someone, discussed the uses of a [Huge Brown Sack] in trade channel, or threatened to sic your homophobic congressman dad on them -- you get three hours or days to think about what you did wrong, and probably plot some revenge. There's also the "temporary permanent ban," which is what Blizzard uses when someone's account is hacked. You're "permanently banned" until they can figure out who really owns the account and how it was accessed. Then the ban is usually reversed, and stolen items are restored. The latter has happened to a couple guildmates of mine, and most were just happy to get their stuff back.
The true permabans are usually reserved for the worst of the worst offenders -- although theoretically, any number of common activities (like account-sharing) can result in a permaban. Speedhacking, botting, gold farming, powerleveling, and of course removing walls in dungeons can get your account banned forever. Note: screaming, threatening, quoting the Bill of Rights, or being hideously rude to Blizzard will not get you your account back.
Have you ever been banned, suspended, or warned for your in-game behavior? Do you have any funny or sad banning stories from your realm? Do you think Blizzard is too tough or too lenient with their banhammer?