Hey, great question!! ha ha.
Answer:
Ignore, ignore, ignore, ignore, ignore. This is called “extinguishing the behavior”.
Sing a little tune to yourself; practice remembering all the words to it; practice your
melismas; try it in other singer’s styles. Practice whistling. Concentrate
on the list of stuff you have to do today; in other words, tune out. Do not participate!
This is a very zenlike discipline. It’s hard at first but enormously fun once you really decide to embrace it. It will drive the annoyer NUTS. Very satisfying.
Remember this mantra: One of two people must change. Which one can you change most easily?
Good luck, this is not easy, have a ball.