I'm not sure which version of the BES you are using but for Exchange vs 4.1 and above you can go to your user policy and look at the following;
Security Policy Group - Disable USB Mass Storage - Enable
Right underneath this option are two other topics mentioned
Disable External Memory
Disable Media Manager
After making changes to the policy that is assigned to your users, you can either wait for the policy to auto apply at a scheduled time, or you can highlight the users then right click and choose "resend it policy"