There is not really much you can do. Well, there is one thing - never make anybody mad enough to do that!
You can not stop it but if you keep stats and email adsense when someone does it they will not bann you.
well... you can use opensource script such as openX where you can limit the number of times an user can see that same advert. though seeing and clicking don't go hand by hand. so like mentioned you can't fully protect yourself just try to keep on top of your stats and contact google as soon as you see something funny
Find a script online, or just pray that it doesn't happen. I've only seen that a few times, but I'm not sure if you'll get banned for that. If you notice abnormal levels, contact google immediately. They usually fix some of these problems...so I've heard.
You definitely want to contact google a.s.a.p. on that. I would also suggest getting a click fraud script like fabriciogr suggested. I like to run a couple websites with adsense, then copy those websites and run them with ypn. this way you can see who performs better and who has better payouts. In case one goes down you can always supplement with the other.
Theres nothing you can do to avoid click bombing, but, adding trackers like aserp will help you to get the logs.