Okay, so I've got a handfull of questions on my mind, so here goes nothing:
1. Is it possible to disallow a LAN-user to forward his/her internet connection to someone else?
2. If 1. is yes, how can it be done?
3. If 1. is yes, how can it be bypassed? So far, the only solution seems to be a transparent proxy server (squid).
4. But what about the other ports (DC++, MSN, Y!), how can thay be "forwarded" and still pass through the firewall?
5. Am I asking too much?