There are two ways:
1. Delete the sending for this customer (in Sendings)
This will delete the feedback and cross-sell replies from that sending
On the Sendings page, search by the customer's email/phone number/name, and use the trash can on the right to delete the sending.
2. Delete the customer directly (in Customers)
This will delete all sendings and all results (except Google reviews) related to this customer.