You need to figure out what is holding it in place. There may be caulk/adhesive on the back side. Get a thin 1" putty knife and work it between the wall and frame, you'll either find glop of some kind that needs to be severed or a hidden fastener you haven't removed yet. If it's caulk the putty knife should cut through it.

It may not be possible to remove it without breaking it.

I assembled my shutter frame but it's not even. How do I take it apart without damaging it