Assuming these are constructed as I expect, you can simply remove the backing piece w/ the hinge and replace it with a plain backing piece — like mat or cardboard.

Or, you can get nice, inexpensive frames from Ikea. They have a large selection in various colors and sizes.

All you need is glazing pins, eye-screws, picture wire, kraft paper and glue to turn those into regular hanging frames. All those items are inexpensive and easily found at a hardware store.

Thank you all for your help. I may try just throwing the back away and inserting mat board. Appreciate your help!