You have a couple of options.
- Go to Options > Security > press scroll wheel and select Wipe Handheld. This will delete any user specific data. Reactivate device on BES.
- Use JL_Cmder (see my signature) to completly erase the handheld software. Download and reinstall handheld software specific for your provider and BlackBerry model (complete instructions HERE). Reactivate device on BES.