Refunding a paid invoice

Former Member
Former Member $organization

We have two constituents who will not be attending a summer program.  Both of the orders were paid for using Invoice.  The invoices were then paid off with a check.  They are now unable to attend and we will be issuing them a refund.  We can return the seats in the order but we also want the invoice to go away.  How can I do this? 

Thanks

Margaret