First, you should think in the opposite way: you should configure your server's firewall to block all ports except those of the services you run on the machine. E.g. for a HTTP/HTTPS webserver that must be accessible via SSH, this is the relevant iptables config:
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 443 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
Secondly, your machine should not even have a telnet server running. It's an unencrypted and insecure protocol; set up SSH instead.
This is just an extrapolation on my part, but I suspect you might want to block external users from using telnet to probe ports on your machine, e.g.
telnet yourhost 25
to probe if there's a SMTP server running. If this is the case, you can't stop them -- just configure the firewall to block all incoming connections except those on the ports of services running on the server.