Stop Unathorised Internet Use

Hi Techies,

I am using a Lan with about 50 users,with xp,win 2000 pro,and one 98 machine,We use DNS to share the internet throught a firewall and modem only 25 approved users can be on the firewall.

because the xp machines loose network resourses when they connected to the internet. we have to gave the approved users the dns so they can manually use it when they want to get on the net and they can change back to assign automatically ip to connect to the network.

the problem is i believe that some smart person has given the dns or is using it to get unauthorised computers on the net.can i stop all the other computers from gaining access to the net even though they were to use the correct dns.

thanks in advance