No PB in front of me, doing the from memory, which maybe faulty.
Under tools, got to accounts.
Find the listing of all your accounts.
tap the edit button (pencil)
Trash cans should appear next to the accounts.
Tap on the trash can next to the account.
You can delete them, but if you click on an email link in a web page, the Playbook won't use your bridged email but will ask to create a native account.