You're not able to delete rentals once they've been added to your Tenant Turner account. This is for both reporting and security purposes.
All rental history is tied to the rental ID in our system, so we keep that intact to reference historical data like which leads came through, how many showings took place, etc. That can be useful to you for legal purposes as well, to protect you in case due diligence is ever called into question.
You can always filter your Rentals page in Tenant Turner to remove any inactive rentals from your view.