Does anyone else have a biting problem?(bad habits)?

Hmmm My gray gelding used to kick out at you when you walked behind him when he is eating his grain. He was very close to kicking me one day and he got a boot in the belly. Also when he thought about it I would let out a loud HHEEYY!! And smack him on the belly or . I eventually got the point across to him with another boot in the belly when he almost kicked my other horse. No he just stands there when he eats his grain.