How to "impede" hinges ?

I have small shutters fixed on the inside an external door. The shutters are very lightweight. They open/close very easily on their top and bottom hinges. When closed, the shutters are held closed by small magnetic catches. The shutter hinges are about an inch and a half long, with hinge pins.
Sometimes, I would like the shutters to remain open - at 90 degrees to the door - but they have a tendency to swing shut on their own.
The aim is to stop the shutters swinging shut on their own, by stiffening the action of the hinges, so it normally takes effort to open/close the shutters.
Any ideas? I think the solution must involve the hinge pins - is there a "trick" for this?
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Place a small sharp cold chisel, or similar, against the folds of the hinge, then tap them in with a hammer to tighten them around the pin. Don't hit them to hard or you might snap or cut something.
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