When "Mobile Device Access" (Administration -> Users -> Users -> <User>) is set to False blackberry device users are still able to log into CRM.
The first step is to identify the exact user string that the blackberry is using
eg: Mozilla/5.0 (BlackBerry; U; BlackBerry 9900; en) AppleWebKit/534.11+ (KHTML, like Gecko) Version/126.96.36.1996 Mobile Safari/534.11+
There is a good article on blackberry user agent strings on their site:
Once you have the user agent string for this device, Navigate to Administration -> Advanced Customisation -> Devices.
Select the option to create a New Device, now add your user agent sting.
The Blackberry device will now be recognised as a mobile device and the users on blackberry will be prevented from loging into CRM.