Forms :: Filter Values Available In Combo Box By Empty Field In The Table?

I'm working on a table for work which will serve as a database of bins and the products currently in those bins, as follows:

ID Bin Amt Product
1 34 25 110001
2 33 15 200005
3 32 23 110003
etc.

Basically: employee A will use a form to update "Bin 34" (ID=1), with information (Amt, Product) until it is emptied by employee B and cleared using a separate form. No new records will be made or deleted, just the adjacent fields (Amt, Product) cleared and filled in and cleared again over and over.

What I'm trying to figure out is, when employee A clicks on the combo box on his/her form to select a Bin and enter information, how can I filter what is listed in the combo box to display only those bins which have no values under Amt & Product. The real-world risk of overwriting a bins fields before it has been cleared by employee B (who physically empties the bin) can be extremely high monetarily so I am trying to reduce that risk by eliminating it from the combo box until employee B has cleared those fields.