I saw it too.
I was quite surprised as we got this letter too and were asking ourself how to block such sites technically. I must confess, we did not find any faisible solution.
=> Blocking the IP-Addresses of the sites. Affecting many other sites hosted on an affected ISP. Many ways to dodge those blockings by means of open proxies on other networks or even using sixxs net via ipv6.
=> Redirecting all traffic to port 80 via a transparent proxy. Breaks many applications like NTLM Authentication etc. Easy to dodge by the same technique as mentioned above.
Still the best way is to notify the affected ISP that they host Child-Porn. I don't think they know about it. If they do, notify their uplinks. I guess they would get disconnected rather quickly.
-Benoit-