I don't believe site-specific blocking can be done at the router level. The closest you could come with an Airport Extreme would maybe be to redirect its DNS IP's to an alternate DNS that ignores the websites you want to block. There are easy ways around that method though (Google caching, etc.) Perhaps if you could provide a little more detail as to what you want to block, where, and to whom we might be able to come up with some more ideas.
I did a search and found OpenDNS. Set it up and added my domain to block. ANd it works ok. I wanted it like Apple has it, like when you go to Myspace in a retail store, it just says it can't find the server, this says, it has been blocked.