Drag and drop - allow one drag item per drop target

Hi

I have a 'simple' drag and drop question. 4 items to drag, 4 drop areas in Captivate 9

However if i drag item 1 to drop area 1 (So drop area 1 is filled) i can still drop other elements onto drop area 1. In other tools you can select an option to say one item per drop area.

Whats the easiest way to do this in Captivate so once the drop area is full you cant drop anything else on it ?

Thanks