While many of us will agree that "asking first" is the right thing to do, I have to say that it works both ways. The OP has spoken with this woman at least once since knowing about the problem and not addressed it directly.
I don't thing putting up more signs is the answer. This indirect communication is just as much a problem as the original "offense." I don't know what the properly layout looks like, I don't know if this woman comes from another area where all the horse people gladly shared each other's trails.
Basically, two wrongs don't make a right. The "neighborly" thing to do is to give her the benefit of the doubt for the first offense and then CLEARLY explain what you want from her. If you don't want her riding on your land, simply say so or have your people person fiancee tell her if you are unable to. This can be done in a very nice way to both keep her off your land and not make and enemy of a neighbor.