I wonder what happens if I have a website with adsense and a user wittingly clicks the ads many times for a long time with result to ban my account. Google bots will trace this behavior and I suppose will ban it but I am not responsible for this behavior. What happen in these cases? Has google any answer for this?
From an advertiser's archived correspondence with Google:
Repeated clicks on the same ad is known as 'click spamming'. We take click spamming very seriously and work to ensure that our advertisers are billed only for legitimate clicks. The AdWords engineering team proactively identifies and filters fraudulent and excessive ad impressions. Once we have identified a 'click-spammer' we take steps to exclude future impressions from that individual on our system. We also make sure that advertisers are not charged for clicks that result from this type of spamming activity. Since the behavior of clickspammers changes, we frequently update our click spam detection to combat click spam.
It is possible that a user would legitimately click on your ad more than one time. For example, the user may be comparison shopping or returning to your site for more information. Additionally, some Internet Service Providers assign a single IP address to more than one user. For this reason, you may see a certain amount of 'multiple clicks'. These clicks represent legitimate users accessing your advertisement in expected ways.
Please know that all clicks that are considered click spam are automatically filtered from reports.
Google claims to be able to detect click spam: in the case of an advertiser, this means avoiding unnecessary billing and, in the case of a publisher like yourself, this means identifying accounts which appear to be generating revenue on spammed clicks (though identifying whom is responsible is virtually impossible, given a moderate level of sophistication on the part of the click spammer, not unlike a joe job).
There would be no way to prove that you weren't the one attempting to game AdSense for your own profit, so I suspect that your AdSense account would be suspended and/or terminated if the fraudulent activity continued.
Google has to assume that you are responsible if fraudulent clicks keep occurring on your AdSense account, however, you shouldn't worry too much about competitors trying to run you out of business by clicking on your AdSense ads because there are so many sites running AdSense that it would be an impractical way to compete.