Drag-drop accept all, but reject once filled?

I have a drag-drop exercise with multiple sources and multiple targets.  There are more sources than targets (some sources are distractors).

All sources are acceptable by all targets (I'm looking for all applicable correct answers, in any order). My problem: users can stack many sources on one target.

I have "redrag" selected, but some users don't or won't redrag. Undo only works on one previous drop action.

Is there any way to set a target to accept only one source, then reject all others?  Something like

If target has an answer then

       reject source drops

else accept source drops

This should work for incorrect as well as correct sources, as I want users to submit what they think is correct without getting feedback first.

Am I running up against a baked-in drag-drop rule? Is there some way around it?

Thanks in advance!