Deleting all from the BlackBerry at once is a useful option if you're wiping the device of data before passing it to a new owner, or if your contact list has become too long to be manageable. To delete all contacts at one time, delete the Address Book database using the BlackBerry Desktop Manager software. You can download the BlackBerry Desktop Manager application from the BlackBerry website for free.
Open the BlackBerry Desktop Manager software on the synchronising computer.
Connect the BlackBerry device to the computer using the USB connector cable. The computer will recognise the device and list the device PIN in the lower right corner of the main menu.
Click the "Backup and Restore" option on the main screen of the desktop manager.
Click the "Advanced" option. A list of all databases will appear in the right pane.
Click the "Address Book - All" option and click the "Clear" button.