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When the ads showing on your pages no longer enhance the content, and the content of those ads would drive away traffic, it is time to block. No amount of cpc is worth degradation of a site.
If you have alot of those and block alot of those, and instead of Adsense ads they are YPN ads, you can expect the ads to show up inspite of being blocked. So, then you pull the YPN off the page, put Adsense back on, and move on.
There are some free services on the internet that specifically test these domain names for bad practices - one of them being a spam test. I noticed that most of these sites, if my visitors were unlucky enough to sign up, were sending 100-200+ spams per week to test email boxes that were set up. I don't want my visitors to ever have to experience that - even if it would be their own damn fault - so I block them.
I have a feeling they pay rather low, as well.
Also, any sites that claim to have found the "top" or "best" x number of sites about a topic - usually block those as well, wihtout question. I want my visitors to eventually land on product pages.