My horse was annoyed with how long my farrier was taking and threatened to kick him. Let me tell you, within seconds my horse was a quivering lil sorry mess of a horse that was backed into a corner and convinced his life was ending.
A slap on the rump is like telling a toddler that just hit someone "oh, that wasn't nice, you shouldn't do that".... they're just going to ignore you and go whap someone else even harder.
Make the punishment fit the crime! Kicking kills humans so the horse needs to think that if he picks up his foot to even THINK about kicking a person that HIS life is going to end.