Over several years of playing this
game in various different
drama clubs, I've decided that the best way to avoid smiling is this:
count.
You can count from one up, or you can count down from 100, or you can perform any combination of mental arithmetic. You see, when you concentrate on the numbers and not on what your "attacker" is saying or doing to you, it won't even occur to you to smile.
The hardest part is halting your concentration long enough to say, "Honey, I love you but I just can't smile." That will take self-control, but you can do it.