In one of my form, I have setup users to select one item in two dropdown fields and when they go up to a certain point in the form, I want to set these dropdown values to ReadOnly to be sure they don't go back and change them.

I have used the following line in the Javascript section:

$("#Field20").attr("readonly", true);

This does change the field to become grayed out but this still allow users to change the dropdown value. 

How can I change the dropdown property to prevent users from selecting another item?