Please! How can I block porn sites by typing a word such as sex key

To reasonably do such filtering you're going to want to use an external solution and use the parent proxy feature of webproxy. Good external solutions are Squid with lists from sites that build filters for you, or Dan's Guardian (though I'm not sure how well that one deals with non-US centric situations).

The built in MT proxy is not going to let you store several hundred thousand lines of rules gracefully.

HTH,
Felix

Is more simple to use the dns offered by openDNS, register the ip address and select interested categories: 5 minutes and it works.

And it's free.

We use open dns as well, it works very well and is external to you and free.