You could use GPO to block it, but there is still ways around that. There are many proxy sites that allow you to access myspace through them, although you could block access to those as well in the GPO. I found it easier to use my firewall to block myspace.com as well as the proxy sites some of my users were finding (via google). I wound up putting in over 100 IP addresses to block to cover this, finally my user gave up. Easiest way I guess would be a proxy server, but I didn't want to go through the hassle of setting that up.