If your riding your horse tell him to halt. If he moves is hind end or takes a step forward or back walk him a few steps forward then halt again. Do not get angry and yank on the reins. Wen he halts for 5 second pat him . Do that for 10 then 15 and so on an so forth.
So let’s just say you’re in the cross ties grooming and your horse starts to move I would discipline him first then I would move him back to where he was standing originally and keep on grooming and if he does it again just keep on doing that till he gets it