Any way to re-enable a drop target once a previously dropped item is removed?

I have created a drag and drop interaction where the learner will have items to either sort or leave in the word bank.  The problem with it is that a learner can drop a new drag item on an already occupied drop target (The worst case scenario is that they drop a correct drag item onto a drop target already occupied by an incorrect drag item, which freezes the drop target and prevents them from being able to correctly answer the question and move forward with the course.)  I could just not freeze (disable) the correct answers so that they can move them to get to the incorrect items beneath, but I don't feel like that is a good solution to the problem.  I also know that I can disable the drop target once an item is dropped on it, but I'm not sure how to get the target back if they subsequently move the drag item off of the target (if they incorrectly drop it, I want to allow them to move it either to another drop target or back to the word bank.)

Any suggestions?  An example of my interaction is attached.