I'm not sure on Linksys firmware, but if you load up DD-WRT, its under "Access Restrictions> Website blocking: By URL, By Keyword" Myspace.com for the URL and "anonymous, proxy" for keywords.
If i had to do over again, I'd definitely done some research and built one from scratch for cheaper, but it is what it is. After nutes and some pH balancing, I've got a couple of good ones.