The Onboarding Maintenance grid includes a Delete button. However, in most cases, deleting a record will mark it as Rejected. This ensures that the history of Onboarding invites and information received is retained.
When the status is Invite Expired or Invite Cancelled, you can click the Delete button. You will receive a confirmation message to 'Reject this Record'.
If the record has a status of either Invite Sent or In Progress, then the Invite needs to be Cancelled first, which you can do from the Onboarding page when you open the record. If you try to delete a record with either of these statuses, you will receive an error that the Invitation needs to be cancelled first.