When you try to cut a piano hinge with a hack saw, you often run into the problem that the hinge pin just rolls around, not staying put long enough to allow the saw teeth to dig into it.
Many piano hinges now have one of the two leafs crimped onto the hinge. That means that one leaf rotates on the pin but the other does not. This makes it easier to cut, and helps prevent the pin from working itself loose.
If you don't have those helpful crimp marks, you can bite the hinge anyplace along it's length with a pair of Vice-Grips to stop the pin from rotating. This may jam one leaf to the pin a bit, but the other leaf will still move freely.
That will allow you to cut through the hinge easily because the pin won't roll.