The event occurred, the charges were all invoiced, but the customer refuses to pay for a special item that never showed up.  You do not have to void and re-create the invoice.  In the Invoice Wizard, select Create a CREDIT – MANUAL Invoice.  Create a credit item, enter the amount as a negative value (e.g. Swan Ice Sculpture = –$60.00) and apply it to the relevant invoice.  There will no longer be unpaid charges for the event.