Do a Security Wipe on the device. Once you have wiped the device, no one will be able to access your information.
How to do a backup and security wipe of Blackberry... - BlackBerry Support Community Forums
For wiping your handheld follow the below steps -
1. Go to Options
2. Security Options
3. General Settings
4. Click Menu button
5. select Wipe Handheld
6. type in
blackberry to proceed