I found a simple, way without resetting, using a basic Windows command. Select the first password, then scrollto the bottom and Shift + Select the last password. That selects them all. Hit Delete. Done
cheers
Probably the fastest option would be to reset the account. Just make sure to back up the old data or be absolutely crystal clear that you want it gone forever, because there's no recovery.
https://lastpass.com/delete_account.php