Some new players who never played tank before might just be ignorant.
If someone turning stance on and off, it's probably a missclick — saw that a few times, and if not, then just tell them they don't need to press this skill all the time.
I've met one newbie Gladiator in Sastasha who was pretty much oblivious about stance, despite finishing Hall of Novice for tanks (could tell that from her gear). Told her about Iron Will, and things went pretty good from here.
Just ask tanks after first pull, and if it doesn't work, elaborate why they need to turn stance on. That's it. 2 minutes of typing or 20 minutes of suffering — it's an easy call.
Problems starts when it's a level 50+ dungeon, tank don't know what a stance is and just doesn't understand any of 4 languages in autotranslate. There, I'm afraid, only option is either leaving or kicking said tank.