I would exclude all of the above. Hang up a sign that states blind horse and you are inviting every last nimwit on the face of the planet that spots it to do stupid things like poke the horse with a stick or throw rocks at it, just to see if the horse sees them. Any information you provide just invites unwanted/needed attention to the horses.
I did not even think that people might do something like that but you are totally right.
Basically my thoughts about this were that there are so many small children that come to visit the horses with their parents and when I was little and doing the same thing, I always wished I knew some information about the horses - for funnsies.
However, the more I think about it, maybe that's not a good idea. Most of the kids in my neighborhood are good, supervised, kids but there is a "pack" of hooligans and I wouldn't want to give them ammunition of any sort...
I guess on the most basic level, I want to make sure everyone knows that they are not to go in the gate without my permission (not that it's been a known issue but just to be sure). They can pet the horses over the fence but no entry. I guess a "No Trespassing" sign would do the trick, "No Trespassing" just sounds so mean to me so I hesitate to do that...
Both mares adore kids/people (and they both aren't tall enough to really reach over the gate) so their behavior is above reproach, it's just some people...
Saskia+Sandy - it's basically the US version of agistment, I think. They're on my neighbor's property, about a 5 minute walk from my house. I'm the sole boarder. Basically the thing is that there's a gravel road right in front of the gate that people love to go for a little walk on and the horses love standing up by the gate because....they do, so they're a bit of an 'attractive nuisance'.
Thanks for all the great ideas! :)