Also these fraud clickers are sometimes hired by your competitor, who is aware of most of your campagins. You loss is his/her gain. Say if you run a campagin with overture and another with Google, these clicker army will click only few times everyday with various combinations.

To explain everything I will have to write few more para, but there is a continues fight between your tracking company and fraud clickers. We studied skewness of clocks, various component identification and many more things to track them properly.