There are several  ways to do this:
1- By Changing standard discussion technique and asking the students to write their short answers down before discussing the question. Then call on with one person at a time.  And .when your problem student violates that process, ask him/her to stop EACH time. by using comments such as “Please remember I want you to wait to be called on”,
 
2. Tell the class that you want them to discuss the question, but “Nancy” is going to review the discussion at the end & let us know if we’ve missed any points. It gives that person a chance to “show off” but still puts you in control of the discussion.

3-  AkE Nancy to facilitate the discussion. 

4-Create a contest  where partipants have to hit a buzzer (or simulate a buzzer) to be the first to answer.

5-Create a round between students where each person takes a turn to answer first.
6- Don’t hesitate to say “Nancy, please don’t answer the question until the others have given their answers.” 