There are many ways to accomplish this task, but first lets find out if you really want to completely block sites like these or if want to restrict access by children, or allow teens access and monitor their online behavior. The latter choice/option being the preferred way to go.
You know as well as I do, if you make restrictions on teens or you stop them from doing something as a form of punishment they will find a way to undermine your authority. They will find a way or sneak online somewhere else. And the possible outcome could be very tragic.
Another option to help teens, is by becoming an active partner in the mySpace craze. Make your own profile find out what all the excitement is about and show them how you would surf and add friends and why you as a parent would also want to make correct choices and be safe (adults need to be safe also).
Teach them how to use common sense and show each of your children the proper way to enjoy the Internet. It’s a great teaching moment!