A few months ago, my sixteen year old cousin got caught trying to teach her grandpa's young mare to rear up by jerking back on the hackamore as hard as she could and kicking her in the sides at the same time.
When my great uncle was told, he banned my cousin from riding any of our horses (his two mares, her mothers mare, and she knows she isn't allowed to ride my horses, so that wasn't a problem in the first place) and has been seriously debating selling the mare she tried to make rear up, because that horse will also pull a cart and a plow and my great uncle isn't physically able to get out and work with the horses anymore. Usually, when he gets mad like that and ''ban's'' someone from the horses, it only lasts a few weeks at most...
But I'm pleased to say that he's held firm in his rule that my cousin isn't allowed to ride any of the horses in our pastures. It's been several months and he hasn't budged, no matter how much she asks.
She's been having to ride her friends gelding, and she knows better than to try and 'teach' that horse to rear. She'd lose those riding priveleges as well as a friend.
I think she's learned her lesson, though... don't try to 'teach' things to a horse that doesn't belong to you and be grateful that you're allowed to ride horses at all.