Not much info here. But I will make a wild guess. When your clients connect, they probably use an account with a regular shell, like /usr/bin/sh. Then you depend on a .profile or other start-up file to send them to your application. If they hit interrupt fast enough, they can break out of the .profile. If this is your setup, you should change it. Make your application their shell. Then they go straight to it and there is no regular shell for them to break into.