Everyone please don’t get me wrong, but I really need to put this answer like this:

Please don’t fool yourself with the idea of blocking anything within name resolution (DNS)!

If you want to block access to / from a network, you really need a full featured proxy and a state-full firewall.
Note that I’m not saying that you need to buy anything, though.
As everyone know, we get what we pay for; but you can achieve a good level of protection with completely free products. (more on that later if you need)

Please post back if you need further help.

Maybe if we know what type of DNS software you are using we can give you better options.
Other thing you must consider is the fact some users can put IP entries in their hosts file if they actually know the IP address of the website you intend to block in order to jump the dns service.

I think a firewall or a content filter are better options.
If you like open source solutions take a look at ClearOS, a linux based solution for enterprises.
Another recomendation could be Watchguard, a paid solution.

Regards.

It will improve general security and help with situations like this.

Do you have ISA? also another good place to block traffic from. You could put an entry in your DNS that refers the address to something else but it’s not advised. I would go with Gabe9527′s suggestion for a short term fix.

If users figure out you’ve blocked it they can still get around this using a proxy avoidance website.