Is it possible to disable autocomplete result?

I have this:

A: 123
A: 456
A: 789

I want that if, for example, A:456 was chosen, so the other options will throw an error telling them that user A is already chosen (with 456). Then, if he wants to change from A:456 to A:123 he has to uncheck the 456 first.

Is this possible?

BTW, in autocomplete results, can I set options groups?