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I was recently banned from adsense and I had about a $100 in my account...Hmm?
I also recently got an email from a friend asking where my ads went. It turns out he was clicking on the ads when he was bored at work. Since Google will not tell me why they banned me I can only guess this was the reason.
The policy does not make complete sense to me, you could really do damage to someones website by simply clicking on a few ads everyday.
Adsense is still the best, however.