SkyConnect supports standard protocols for services, which are used by the clients, both desktop and mobile. Since the last mile connection to the services happen over the same protocols, irrespective of where the connection comes from (desktop, mobile or web), it is not possible to enforce a policy for disabling mobile access for certain users.
The workaround to this is to allow such users only web client access and disable their POP/IMAP completely. This will ensure that these users cannot access their mail either from mobile or from desktop clients like Thunderbird or MS Outlook.
User set 1: Web client access only, no mobile, no desktop client access
User set 2: Web client access, mobile access, desktop client access
User set 3: No Web client access, mobile access, desktop client access.