I would think if your Subcontract Resource was set up with finite capacity, it should only be scheduled for one operation at a time. The subcontractor would have to finish one operation before it began working on the next.
I would also look at the “Concurrent Capacity” field on the Resource. I think it defaults to 0, which I assume means infinite. But if you changed that to (1), I think it would limit that resource to be scheduled on one operation at a time (not concurrent).