Is there a way to limit Updatable field on Dashboard to only use UserCode data

We have an updatable dashboard where we added a UD Field and then we created a UserCode to populate the dropdown list.

This works fine - except that the destination field is a nvarchar that will take any input and if users do not use the drop down (or override it) then they can put in any info they want.

We would like to limit the field to only values from a BAQ.

Is there a way to do that?