I'm not sure if blocking a specific URL is the solution... it seems trivial to mirror content on the internet, and so any attempt to block specific content can't be accomplished via black-listing specific URL's... but even if real filtering of data were technologically feasible, it's completely defeated by end-to-end encryption like SSL, which could easily become standard if ISP's start inspecting your packets for "black-listed material".

So moral issues aside, this is a huge waste of effort no matter which way you turn it.