filewall
so you know all the IP's of your account holders? What kind of server are you running? You can set up your firewall to only alow your "trusted hosts" to get in. Also, depending on what kind of server you are running, your serverApp might beable to block some people as well. You should only have the services that you need to be exposed to the outside world.
ftp, ssh, Hotline, Carracho, KDX, ...???