what is the good way against reverse telnet?

["Monty Ree" <chulmin2 () hotmail com>]

Hello, all.

I have operated firewalled linux webhosting server.
But only input traffic is secured and outbound traffic is totally opened by 
firewall.

So some attacker attacked these servers using reverse telnet.
So is there any method in order to protect against reverse telnet?
Any method or solutions without prohibitting outbound traffic?

Thanks in advance.

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