Combating reverse telnet using OpenBSD Packet Filter (pf) – This paper goes into detail on how to set up a firewall properly to disallow outbound traffic from reverse telnet attacks from compromised machines.
You can download it from the following link: https://packetstormsecurity.com/files/download/30526/wan.txt
Source: https://packetstormsecurity.com/files/30526/wan.txt.html