Drag and Drop Issue - How to disable drop targets

I have exactly the same issue. (See attachment)
I can't disable drop targets.

I have two sides, 7 objects and 7 drop targets.
4 objects must go on the left side, no matter in which drop target. 3 objects must go on the right side, no matter in which drop target.
When I place an object in a valid drop target, the object is disabled and it is no longer possible to redrag it. When I place a bad item over it, it takes the place of my disabled object and the first one returns to its original position but remains in the disabled state. Which I don't want.

I would like the drop target to be disabled when a good object is placed in it, so that it cannot receive other objects.

Given the number of possibilities per drop target, if I use layers to replace each filled drop target, I have to create images for each possibility, which makes me a large number of images and layers (4 images per drop target on the left = 16 + 3 images per drop target on the right = 9).

Any ideas?

EDIT :

Even if I "hide" my drop target and replace it by another image, the drop target is still active.