How to block certain website

You can get commercial products, Websense or N2H2, which the PIX will work with to do filtering. These are expensive -- unless you are a company that needs this kind of facility, in which case they are a cost of doing business that could end up saving a lot of money.

The filtering capabilities built directly into the PIX 506 are limited to IP addresses and ports, not site names. Thus if the users use a proxy service, even a trivial non-encrypting one, then the PIX 506 would not be able to detect that. And if the web site to be blocked happens to share IP addresses with other sites, the PIX 506 cannot look to see which name was requested so as to allow the "good" sites through but not the bad sites.

The highest software version officially supported on the PIX 506 is 6.3(5), which does not support timed ACLs. If you open up the PIX 506 and increase the memory in it, then you can get PIX 7.0, 7.1, or 7.2 to load into it and run, and those support timed ACLs. PIX 7.x is not officially supported in the PIX 506... and you would need a support contract to have access to PIX 7.x.