Overflowing drop zone

Hi all -

I've built a simple sorting activity and am having difficulty preventing the user from stacking all 7 items into a drop zone that is only big enough for 4 items.  I have not encountered this before, so I'm completely unaware of how to prevent it.  I see no setting for controlling this behavior, so I would be grateful if one of you could point me in the right direction. 

More about the interaction:

  • there are 5 phrases to sort (see blue column in screenshot below)
  • there are 2 drop zones (RELEVANT and NOT RELEVANT)
  • see 2nd image below to see what is happening

Any ideas on how to prevent?  Thank you in advance...Paula

Initial screen

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