In Akti there are two possibilities to "delete" credit invoices:
1) The credit invoice has already been sent to the customer: in this case you have to create a new invoice to equalise the balance from an accounting point of view for both parties. So strictly speaking, this is not really a deletion of the credit invoice but rather a cancellation.
2) The credit invoice has not been sent to the customer yet: you can just reset the document to the draft status and archive it as you do for other documents in Akti. More info on how to delete/cancel documents in Akti can be found here.