That's possible, Ricci. Even with No Trespassing signs though, they don't sound like they understand something I learned in kindergarten: If it's not yours, don't touch it.
I didn't see where the landowners were even mentioned by the OP, so I don't understand how it came up that they weren't being supportive. If the OP is renting the land, I'm not sure how it's the landowner's responsibility to make the neighbors behave themselves.
OP, as I stated before, you might want to contact an attorney concerning this matter. If you have pictorial proof that they were riding your horses and you confirm that you never gave them permission, the attorney can at least send a cease and desist order.
As the horse owner, you're liable if someone gets hurt on your horses. Yes, I know that stinks and it sounds bass ackwards, but horses are considered 'attractive nuisances', so you need to have all your ducks in a row legally.
I'd also put up motion sensor lights and wild life cameras, so you can catch them in the act if they try and do it again.