Once a ballot has been created it cannot be deleted. It can only be archived.

Ballots can be archived from the Dashboard screen. Click on the archive icon to the right-hand side of the ballot.

Ballots can also be archived by accessing the Ballots tab then locating your ballot under the respective stage it is in (Active, Under Construction, Completed).

All archived ballots can be accessed from the Ballots tab under Archived. Where you can also un-archive those ballots.

Refer to this link for instructions on deleting voters.