Click fraud is a major concern to most advertisers. While advertisers should monitor their PPC activity and be aware of any possibilities of click fraud, there are several tools within Yahoo Search Marketing accounts which can help to protect advertisers from potential click fraud.
You have to make a case. They will tell you that they are researching but will never report that something was found. They will credit your account with a good will credit should you present a case with evidence. Build your case with server logs and report how many clicks you think were suspicious.