It is true that connections to remote X Window servers should be always made over SSH. SSH supports X windows connections. So my task was allow X over ssh but block unprivileged X windows mangers TCP ports.
The first running server (or display) use TCP port 6000. Next server will use 6001 and so on upto 6063 (max 64 X managers are allowed from 6000-6063).
So assuming that you are going to force user to use ssh for remote connections, here are rules for IPTABLES (add to your firewall script):
iptables -A OUTPUT -o eth0 -p tcp --syn --destination-port 6000:6063 -j REJECT
iptables -A INPUT -i eth0 -p tcp --syn --destination-port 6000:6063 -j DROP
a) The first rules blocks outgoing connection attempt to remove X windows manger.
b) The second rule block incoming request for X windows manger. By using --syn flag you are blocking only connection establishments to the server port.
This is the good way to disallow unprivileged X windows mangers - TCP 6000:6063 ports :)