The selection locking feature was removed in version 4 so I went with a

solution suggested on stack overflow - namely, as each selection tag is
rendered add a custom class if it's the principle user.

Relevant posts:
  https://stackoverflow.com/questions/16564425/how-to-implement-select2-with-lock-selection/48111387#48111387
  https://jsfiddle.net/o3s3y42e/4/