Even though this thread is super old, just to add - if a horse thinks about biting me, they also get to think they're going to die post-biting-thought.
If a slap doesn't work, I won't hesitate to break out the big guns (usually a crop/lunge whip - something the horse already "respects". With my mare I'll basically "lunge" her on the lead rope - telling her "GET OUT OF MY SPACE", like a dominant mare would. She tried snapping once a few years ago - did that - she has never snapped again) to back up what I'm saying and we repeat, repeat, repeat the activity that got the biting response until the biggest response there is is perhaps a slight ear pin.
It might seem mean but in a herd, the dominant horse isn't going to stop pinning her ears at an underling because it "didn't work". She's going to escalate until that less dominant horse backs off. Of course, horses and humans are two different things but those rules still apply - horses very much understand escalating "pressure" since, as herd animals, it's how their minds are wired.
Just my $0.02
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.
Last edited by Wallaby; 09-21-2012 at 05:44 PM.