Well I don't know if this is what you had in mind, but I use the web site filter that comes with Trend Micro Internet Security. It lets me block the pages I choose to based on categories and also specific web sites. you can also password protect the program so that others cant change the settings.
I use the filter to block sites like porn, violence, unsafe (virus/problematic sites), file sharing, etc...
It prevents pages that could be dangerous from opening. The program also blocks TONS of web ads from even being able to display. All I see in place of an unwanted page or advert is bright red text telling me that I can enable the web site if I choose to. And the address is shown for if I do choose to add it to the approved list.
Maybe this is what you are looking for, or something like it. I know that there are other programs out there that work in much the same way. I have seen several at Fry's Electronics (for instance) listed under "security software/parental controls". Manufacturers call their software "
parental controls", but it should work in the way that you need.
If you have a business, this is definitely a smart thing to do. I would recommend that you try to find a software that runs as a hidden process. That way you can guard against people seeing the software and stopping it. Just remember to be attentive to enabling or disabling sites based on employees activity. And dont loose the password! ha ha ha
Here is a little article from PCMag.com about those types of software, if you choose to go in that direction. It was written May of 2009, so relatively recently.
http://www.pcmag.com/article2/0,2817,2346997,00.asp