I would post the no trespassing signs, contact the local law authority and get it stopped before you find out more and more people are stopping by.
BUT, do not post about agressive dogs, or before you do find out the current laws.
In most states,ff you post you have agressive dogs, then if someone gets bit on your property, even with the signs they can sue you because you are acknowledging that you have mean dogs. But if they come onto your property and are trespassing, then you stand a better chance of not getting sued for the bites since they had no business on your land.
If this was my situation, I would be bringing the police/sheriff into it immediately. Obviously since she had been told before that she is not welcome on your property, she is ignoring that and doing as she pleases.. Get the law involved before she or one of her cronies comes back when nobody is home and one of your horses gets hurt, or your friends horse or stuff gets stolen.