Queries :: Access 2010 - Return All Products Not Associated With A Variable

I use access 2010. I have a form that allows the user to assign products to operations. Each operation can produce multiple products and each product can be produced at multiple operations. I manage this many-to-many relationship through the use of a third table that stores primary keys for associated products / operations.

tblOperation
tblProducts
tblOperationProducts

The query im trying to design will populate a listbox on the form that contains products which are available to be added to a given operation. In addition to the listbox there is also a combobox that allows the user to select the operation they want to add products to. As the user selects an operation, the query behind the listbox should return records for all products except the ones that have already been assigned to the operation selected in the combobox.

So far I've been able to make the listbox return all products always or return products that haven't been assigned to any operation at all, but I'm struggling to formulate the logic in my mind on how to show only products that have no association (through the tblOperationProducts table) with the operation selected in the forms combobox.

This is the query I have right now, it's not working:

Code:
Select tblProducts.ProductID
From tblProducts
EXCEPT
SELECT tblProducts.ProductID
FROM tblOperation RIGHT JOIN (tblProducts RIGHT JOIN tblOperationProductMM ON tblProducts.ProductID = tblOperationProductMM.ProductIDMM) ON tblOperation.OperationID = tblOperationProductMM.OperationIDMM
WHERE (((tblOperation.OperationID)=[Forms]![frmOperationProducts]![cboOperation]));