How can I remove the screw from a double demountable hinge?
It looks like a lockring is used to hold the screw in place. You'll have to hold the screw still and rotate the lockring, or vise versa. 

Using a flat-head screw driver and mallet, is a common way to rotate lockrings.  Though, you'll have to find a way to hold the screw steady, while you bang the lockring free.


Place a flat-head screw driver blade against one of the sprockets of the lockring.
Sharply rap the handle of the screwdriver with a mallet.




It's possible that the lockring is made of some type of softer metal, so grabbing it with pliers could easily damage it.  Using a screwdriver and mallet applies force only in a useful direction, whereas pliers apply non-useful forces required for gripping. These non-useful forces may damage the lockring.