So basically It will be easy with Reference Selector
Lets think that you are using An Entity with Products which has association (1-*) from your primary entity
So create a boolean (Default value : false) in Products entity , And as soon as the user , saves the data with respect to one object , change the boolean (true) for that particular Products
Meanwhile , Write an Xpath for products in the page where you are saving
Your Xpath should look something like (Isempty=false).
So by this way only the false Products (Which are not selected)will be visible in the dropdown