Using a dropdown control to populate a text field - exclude existing value in field

I learned from this post how to populate a text field with a drop-down control using a custom field renderer, and I’ve had nearly 100% success. My only problem is this: the fields we are now populating with the drop-down control contain some invalid values. Where this is the case, the existing value from the record being edited appears in the drop-down control along with the valid values we populate into the drop-down. I would like to exclude the existing value from the drop-down control in all cases and allow our users to select only values that we determine to be valid. Can I do that?