While there is an easy way to "deactivate" control once selection has been made, I question the applicability of such approach. What happens if a user makes a selection by mistake? Are you denying him an opportunity to correct it?
Let me explain my problem further. I have written an HTML template with about 8 drop-down boxes to facilitate an easy bullet-form description for the seller to list inventory items. Once the complete selection of all boxes has been made and the seller has reveiwed the information, I want to be able to disable all the drop down boxes so it can't be changed by the person buying the item.