Prevent Adsense Theft by using “Allowed Sites” Option
To tackle Adsense Theft, Google has introduced a new feature which lets you whitelist the sites which show your Google Ad. You can specify the whitelist by logging into your Adsense account, and navigating to Adsense Setup > Allowed Site (google.com/adsense/publisher-whitelist-view).

The whitelist doesn’t prevent other sites from showing your Google Ad, but the impression and clicks won’t be count. This prevents inappropriate sites from stealing your Adsense codes and placing it on their site, with the intention of getting you banned out of Adsense.
Not all Adsense publishers are in favor of this feature. Amit Agarwal writes that he’ll leave this feature untouched because a part of his traffic comes from Yahoo/Google cache. By turning on this whitelist, he may be losing a part of his revenues.
I have never been into a situation where my ad code was being used by some malicious site, but I’ll still add my site into the whitelist. It’s pretty hard to figure out, if any other site is showing your Google Ad, so better be safe than sorry.