DNS filtering

Something like that, though that's far oversimplified. Also if this is in regards to SOPA / PIPA, it's also not that easy to perform, seeing as the filtering tech they want ISPs to use isn't fully compatible with what most ISPs are using right now If we're talking about registrars' ability to edit dns records ... well yeah, easy as fuck.

DNS blocking or filtering is a common method of denying access to certain websites. Let’s have a look at how it works.

Each website is hosted on a web server that has a IP address. For example, the IP address for Facebook is 69.63.189.16. If you type those numbers in your web browser, you will arrive at Facebook’s website. Try it – Welcome to Facebook - Log In, Sign Up or Learn More.

However, IP addresses are not very user friendly. It’s easier to remember facebook.com than 69.63.189.16 isn’t it? Therefore the inventors of the internet also created a phone book called the Domain Name System, or DNS.

The DNS translates domain names into IP addresses so that you don’t have to remember random strings of numbers. Each ISP (e.g. Streamyx, P1, etc) have their own DNS servers that functions as phone books for their subscribers.

Whenever you type a website address into your browser, your browser first asks the ISP’s phone book what the IP address for that website. Once it’s figured out the IP address it will then load the website for you.

With DNS blocking, the ISP is simply removing the record for the blocked websites from their phone book. So when you try to load one of the blocked websites, all you get is a blank screen in your browser because it doesn’t know what the IP address is.

More Information: What is a DNS block and 3 ways to get around it » adventures of a blogjunkie