What do you not want to use a firewall? That is the only effective way to block
a port [ip filtering and ipsec filtering are in the same vein]. You can prevent a
port from "listening" for connections by disabling the associated
application/service/process using it, but many times an unknown trojan will open a
port to connections. Use netstat -an to find what ports are listening/connected. Use
something like fport or tcpview for more detailed info on what application/process is
using a particular port. -- Steve