Activity Guide - Delete Tasks does not delete Instances, How to fix?

I had inadvertently assigned the SF Agreement to some students more than once. I ran the delivered Delete Task batch process to delete the duplicates, not realizing that this did not delete the specific instances which remain in the Agreement table. This is causing an issue because the students are now seeing the deleted agreement instances and the actual completed agreement instance, under Completed Agreements, once they have actually successfully submitted the agreement.

There is no delivered 'easy' way to delete instances on an individual basis, and no way to delete in mass. Has anyone figured out how to resolve this issue?