ofcourse: Well, so much for being dan lekaf zechus. Obviously, the way to get them to stop is to hang a sign saying it’s assur to hang signs on other people’s property without permission!
But to address the OP’s original questions: re the signs’ effectiveness: The signs will affect very few of the women at whom it is aimed. Some might stop and reconsider their outfit or even their wardrobe, but for most who dress in a style-conscious way, unless something is said to them directly–in a pleasant way they can be mekabel–or something happens that shows them it’s better to be tznius, it is unlikely that they will change.
re the signs’ legitimacy: no, they don’t belong on trees. Or on lampposts, or construction site walls, or mailboxes, or bus shelters, unless the ad space has been paid for.