iptables -A INPUT -p udp -m string --algo bm --string "BANNED_CDKEY" --sport 20700 --dport 28961 -j DROP
iptables -A INPUT -p udp -m string --algo bm --string "INVALID_CDKEY" --sport 20700 --dport 28961 -j DROP
That should only affect the cracked server on 28961?

If that is not the case, I can hook up the C/recvfrom()-function to block those 2 packets, but then you need to either rent a server @ killtube or buy the extension for 40 €.

@ Ni3ls, Mitch's cracked server is advanced, so it's in the master list and normal/cracked ppl can play on it.