you have to tie them up somewhere for a long time, like in different places so they get the idea when the line is down they have to stand. My trainer taught her big horse Vic to do that, she said she would tie him to random stuff and leave him for a couple hrs. Now you can just drop the lead or reigns anywhere and he doesnt move.
That's what I did with my gelding too, for the most part he ground ties, but he'll still sneak around behind me bc he has to b everywhere I am.
"The horse you get off is not the same as the horse you got on; it is your job as a rider to ensure that as often as possible the change is for the better."