javascript drop down boxes

I am trying to create a kind of quotation calculator, which is based on various elements. It starts by choosing a product from a dorp down box and choosing attributes from checkboxes, however to select another product from a different drop down box again i want to only display the products that have not been chosen already?

In other words i need to create a drop down box that is populated with attributes that have not been previously selected?

ie...

dropDown1 = att1, att2, att3 - selected is att1.

dropDown2 = att2, att3 - selected is att2

dropDown3 = att3 - selected is att3