Neighbours claim our land — how to stop property disputes?

My grandmother bought a piece of land, which was passed to my father and uncle after her death. My uncle passed away, and now the land is jointly owned by my father and my uncle’s two sons. We want to sell the land, but nearby residents are claiming it as ancestral land and have even removed my father’s name written on the wall next to the property. We have all the registered documents in my grandmother’s name. What legal action can we take to stop these disputes?