So far I have not seen or heard of a filter that the pornographers cannot eventually get around one way or another. However, there are ISPs, such as integrity.com, that only allow access to web sites their censors have previously visited and approved. If you want access to a site not on their approved list you submit the URL to them and they make the decision whether or not to allow you access.
Even this draconian level control is not perfect since you can, in some cases, go to a web site and then follow a series of links from that site to others and eventually get to an XXXX rated site.