Restrict net access with pass or similar

Hi all,

I have a network with 50 PC all getting IP, Gateway and DNs via dhcp. The problem that I now face is that since we have an open network due to the need of some outsiders connecting to it to print and so on, I cannot restrict them from receiving an IP. I would however like to restrict them from accessing the internet. They cause excessive traffic downloading and slow down other peoples work. Is it possible to restrict internet access using a passowrd or some other method.

We have a windows small bussines server 2003.

Thanks in advance.
P.