This is true, but typically beyond what a home user can figure out on routers I've used from Zonet, Netgear, Linksys, etc. Heck, most home users don't even know how to figure out their router's IP address and leave the default login and password. The ones that spout about filtering have a web service (remote) that you pay for and use a nice obvious login to configure.
Edit: I should add that IP blocking is generally totally ineffective compared with classified blocking. Seriously, does myspace.com rely on a single IP address? I doubt it. Also, there are countless myspace-like sites. A good filter has something like "social networking" as a class in its database (or porn or violence, etc.) and you can block them all with one click and then add exceptions if needed.