You can delete a card sort by clicking on My Students and then navigating to the specific report you wish to delete. If it is a completed card sort, you will see a screen like the screenshot below:
You can then click on Delete or Archive.
Card sorts that are in a Paused state can only be deleted by the user who paused them. If the Paused sort is yellow, click on My Students, then the report you wish to delete. The Delete button appears below the student's name. If the Paused sort is orange, your student will need to either complete or delete the card sort themselves.