Our seven year old gelding started showing at about 5mos. He will stand tied for hours if necessary. He will also sit down to break a rope or halter if he figures he's had enough :) He'll stand tied to the trailer all day at a show and break loose on a hunting trip as soon as you close your eyes. He learned after he grew up that not much holds his 1200lbs if he sets his mind to it. He also learned best not do that in front of dad :)
LOL.
I know an old man that had a stud that would set back and break leads every time. He finely tied him to a metal post with several ropes to several halters, and sat back and watched. Took something like 2 days before he saw slack in the lead rope. But that horse never did that again.
I don't know that I agree with that method:), but when they get full grown and have a habit like that, I don't know any other way you could break them of it. I wouldn't keep a horse I could leave tied up.
Becky