How to stop users getting mail via VPN Urgent 500 points

London                          Firewall                         Firewall                      Aberdeen
192.168.1.1                   192.168.1.10                 192.168.10                  192.168.0.2
                                                           VPN
                                                                                                           Mail Server



I have the following set up for certain reasons and it cant change.

How do i stop desktop users from London obtain there mail  via Outlook?
I have blocked port 80 on the London side firwall. THis dosnt work as the VPNs route the mail internally.
I cant uninstall Oultlook not an option.

How do i stop them RX mail.

The reason for this the users in london use citrix and collect there mail from citrix.

As all users some have fiddled and found the route via the vpn to collect mail with out citrix. Causing my Bandwidth loss.

Thanks