There have been several times this has happened to me. People just stop by... they looked up our address and decided to just come have a look. (We are off the main road and do not post signs) A couple pointers... I first tell them I really appreciate their interest and would LOVE to show them the horses.... but unfortunately, I'm unable to do so that day. I explain that I want to allow them time to visit the horses and that isn't possible right now. I give them a business card and ask that they either make an appointment to come back or call me to do so. Regardless of who stops in... the first question out of my mouth to any visitor is "Where have you been before coming here?". If they've come from home and have no other horses, no problem. But if they've come from another farm or especially a 'county fair', show, rodeo etc.... they must come to visit with a change of clothes, disinfect their shoes and wash their hands before they ever see the inside of my barn, pastures or interact with my babies.
Picking up someone else's germs could run right through a barn of youngsters.... meaning sick horses, vet bills, meds and a general pain in the you know where. I don't care if it hurts their feelings or insults them. If they are an animal lover, they will understand and if they are really interested... they'll come back. : )
I would do the same courtesy for any other farm I visited.