Neighbors nuisance

We live in Chennai with thatched roofed house. My parents are not well educated and they bought this land few years ago. We are not able to build a good house here. Problem here is my parents believes that the neighbors built their house in our land too by extending their walls. My parents were not aware of what is happening. They still continue to disturb us by throwing cigarettes and crackers on the house and creating nuisance by spitting throwing babies nappies at our house. We are not able to tolerate these nuisance by them. They come into our land just like with no permission when no one is there at home and do their works. We spoke to them many time no not do all these. Totally fed up of talking to them nicely and I don't know what else I could do. Can someone help me on how to know that how much land they have occupied from us and what can be the legal action taken against them for all the problems they are creating to us for years? Or nothing can be done on this and I'm missing something? Not forgetting to mention that they are well sound family in our area. Awaiting for help.