Locking a dropdown after selection

[Misery@Work]

All I need to do is lock a dropdown after the choice has been made. It's not dependent on any other selections whatsoever. They just want you make a choice from the dropdown once, lock that dropdown and continue on with the rest of the form. Regardless of whether this is a good idea or not is irrelavent as I'm just doing what I've been asked to do.

Surely there is a simple solution for this that I'm just not getting. I'm not a programmer and know just enough to get myself into trouble. 

Suggestions?

[try67]

As the custom validation script of the field enter this code:

event.target.readonly = true;