I never realized how bad fraud can be until I swapped out my old Google Adsense code for new. For awhile Adsense reported $200 a month in estimated earnings which was more than the $20 or so I usually get. Then after Google finally corrected for fraudulent clicks I actually got paid my normal $10-$20. Eventually the estimated earnings start to account for fraud so I no longer see $200 projections. That means a majority of the actual clicks were determined to be fraud. Either that or Google is shaving clicks. I lean more towards to first scenario.

So Google already detects and refunds advertisers for fraud.