How to avoid people search in role-based-condition?

I have a dropdown work item attribute called as "BA Contact". It is supposed to show only the people who are Business Analyst. To achieve this I added a Role-Based-Condition. It works correctly, that only the users with that role are seen in the dropdown of "BA Contact". But it also allows me to type in a name, and if I type any name that is not having that role-used-in-condition, it still show up...

How to avoid this?

I want that only the people with that role should be listed and no other user should be searchable in that field value.

Please let me know.