Opening the 7dtd Telnet port to the whole internet may pose security issues as the connection has no protection. The least that could happen is people doing random stuff on your 7dtd server but it could also be that attackers find security holes in the engine and gain further access to the host.

Therefore you should block the ports used for Telnet by a firewall.

To block all traffic on a single port issue the following command:

iptables -A INPUT -eth0 -p tcp --dport 8081 -j DROP

This will drop all TCP packets that are received through the network interface eth0 for the destination port 8081. Adapt the port and incoming interface to your needs. If you want to block a whole range of ports, e.g. because you assign the ports 8900 to 8999 for the Telnet interfaces of multiple 7dtd instance you might simply put "--dport 8900:8999" in the command instead.