As an Admin, you have the ability to delete past 1:1s. Please note that managers and their direct reports are not able to delete 1:1s, only Admins.
Step 1: Go to the Admin panel and click on the 1:1s section.
Step 2: In the Auditing table, find the 1:1 you would like to delete, click on the ellipsis next to it, and click "Delete 1:1".
Step 3: Confirm that you want to delete this 1:1, then click "Delete".