I have the same issue with an MCO-CPH half FFC. Not only am I stuck going back to CPH but the credit expires one year after the date of the first flight. So, I have basically a free flight to CPH (or an enroute connection) that goes away in about a month. When I called they said that part of the problem was the partner segment. They said that in the future that I should not cancel the flight on line but to call them. They could then delete the partner segment before cancelling the return and that this would allow more flexibility in the use of the FFC.