Actually there is no way to disable it completely. In uddeIM 1.1 it is possible to block certain groups to get emails from the public frontend. These users are not listed in the user list, e.g. when you allow sending mail to the superadmin only all other names are not listed in the user list. You can also suppress the "Show user" link for the public frontend.
But indeed it may be a nice feature to disable the userlist when called directly e.g. from a menu only. I add it to the tracker.
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer