question Groupon cpc in AdSense

Hi there,

I have been noticing a decrease in CTR and revenues on some adsense channels. What they have in common is that they are repeatedly showing ads from groupon.

They're niche articles and usually the ads were very specific and CPC was good. I know that google target the advertisement based on webpage context and user's interest but it seems strange such amount of groupon.

Does any of you have an idea about the CPC generated by groupon ads? I am tempted to block them.