That's an unfortunate situation. And while, you've obviously already figured out that the cruiseline doesn't owe you any other type of 'credit' they should be rebating the port fees.
This happened to us and the port fees were reimbursed as shipboard credit as Nancy says, so check your bill just in case they applied it, and you didn't notice.
Princess actually put letters in everyone's mailbox explaining the rebate and then sure enough, it appeared on the statement.
RCI Sovereign of the Seas 2005
Caribbean Princess 2005
Carnival Paradise 2004