Like I said, did they receive NOTHING for the F-15E? Have they publicly said that? If that's the case, I'd be more understanding of them stopping future work.
Otherwise, no, bricking or leaving the product bricked on purpose with the intent of using your customers as bargaining chips is pretty horrible.
E: To be clear, I would be understanding of them stopping future work if they have not been paid, I would still consider leaving the F-15 broken to use as a bargaining chip utterly reprehensible and honestly I'm already filling out my refund so that I can walk away from anything Razbam I own. There are plenty of other aircraft I enjoy flying, I have zero desire to support a company that thinks myself or any other customer should be used as an unwilling negotiation aid.