Google always takes necessary step to protect the interests of Adword advertisers. They have unique system to detect the invalid clicks and click frauds. When they find that their Adword advertisers are at significant risk for a particular AdSense publisher, they disable the concerned AdSense account. You know Google only count the clicks as valid that must be result from genuine user interest. There are some publishers, some how who have managed to approve the AdSense account but do not have enough knowledge in ad networking. Even they have not read the AdSense program policies and Terms of services. They click on their own ads again and again to boost their earning. Clicking on own ads for any reason is prohibited and Google takes no time to ban their account. Some smart publishers think that they will not be detected if they generate clicks from different computers. So they generate clicks from multiple computers using office computer, library computer, friend’s computer or from computers of different cyber cafés. Some times publishers take the help of software to generate clicks or contact other publishers through email, forum or groups and generate click for each other. There are other illegal ways also. But always remember Google is much more smarter than the cheaters. They have several click protection systems and examined by various data points such as IP address, click timings, pattern associated with every click to detect the invalid clicks. Invalid clicks are detected automatically and filtered from reports. Not only that. After filtering maximum click frauds they examine individual instances of invalid clicks by an expert member. So cheating with AdSense is next to impossible.