If both lists are using ArrayCollections for the
dataProviders you could do a check when the item is dropped onto
the smaller list of the length of the ArrayCollection. If it's <
7 ArrayCollection.addItem(object) else
ArrayCollection.setItemAt(object, calculatedDropIndex) to replace
the item instead of adding a new one.
Perhaps in your drag n drop handler you can simply access the
dropped data, and take only the first 7 items in the dropped data.
You could even popup an alert telling the user only 7 items are
allowed, and the first 7 were copied. Then they can specifically
drag 7 they want to copy if they desire.