How to remove domain list from Horizon View Client Logins

For security reasons we would like to remove all mention of our internal domains from the Horizon View Client login screen (ie the drop down menu on the login page).

I have used the vdmadmin command to remove all domains (including the primary one which is the only domain our users would actually login from), but the single primary domain still appears in the list.

We would prefer our users to have to login with a text string of <domain>\<username> or <username>@<domain> and to show no domain in the drop down box of the login screen.

Is this possible, and if so, how do I achieve this?

Thanks and Regards

Gary