Find yourself a good stout oak tree with a limb about eight feet off the ground, preferably with sand or grass under the limb. Fit your untieable horse with a brand new, doubled and stitched, nylon halter, the kind with stainless fittings. It should fit snug, but you don't want it indenting the skin anywhere. Put the halter to the horse, then tie the halter shank to the innertube with a bowline, with about an arm's length of slack in the shank.
School is now in session.
Give a set of cymbals to the most energetic of the barn urchins. Encourage the local motorcycle club to visit the stables. Dobbin finds discomfiting are a Good Thing. Dobbin will discover he is no longer in control, he is tied. The harder Dobbin pulls on the rope, the harder the rope will pull on Dobbin.