I haven't had many problems with strangers and our horses, but uneducated relatives, yes. Most of my horses were given far too many treats in their former homes, and I've tried to break them of being mouthy, and I never give my horses treats anyway. I had family members come over and be petting Takala, and they'd let her take their entire hand in her mouth! If I told them not to let her do that, they just replied "no, it's okay". I've had to tune up others because they'd let Tenakee lip their hand for about an hour looking for treats because it "felt cool".
People don't understand that I don't give my horses treats not because I'm worried about them getting fat, but because I don't want them to maul unsuspecting, under-educated victims. Tenakee got upwards of 15 treats the day she had her foal, and I was not impressed. Not that many people need to be visiting a day old foal anyway. I go out of my way to destroy treats, yet new ones are constantly bought. If you train a horse right, you don't need treats to win them over. Food doesn't merit you any respect.
I am a little protective of my horses, and definitely don't have interest in dumb people who won't listen! My blood is boiling just thinking about it. If I were you, I'd quit my job, by a lawn chair and a gun and sit in my horse's pasture to ward off any naysayers. But then again I am sick and waiting for the sleeping medicine to kick in, so I may be going a bit overboard.