you can do this, we can do this using iptables as well as using squid also.
You will have to block the ports used by kopete for certiain ip (aka user's computer) .
Is that you want to know ?
You can do like
# This line is to drop TCP port 5101
iptables -A INPUT -p tcp -s <ip of the user> --dport <port> -j DROP
here give ip of the user and give the port used by that application. However now a days, most of the application have the ability to use other port, even protocol if one is unavailable. So you have to find out that what resources are used by that application and then we can block it easily.