Filtering Junk AdSense Clicks:

Click fraud is a huge issue. Google now allows you to filter out sites that you
do not want to advertise on. If a site sends you significant traffic that does not
convert you can block their URL so your ads do not appear on their site.

This tool, which I have not yet tried, tracks which sites your AdSense ads are
appearing on. Sometimes low quality scrapper sites actually convert well since
people only find them via search and immediately click an ad while still in
search mode. You can’t determine how well a site will convert without tracking
it, although if you are getting a ton of clicks and suspect fraud then it is best to
block the URL.

If you enable content syndication (displaying your ads on AdSense) make sure
you filter out regions which have notoriously high click fraud (India and most
of Asia) unless you are trying to target those regions.