Land dispute

Sir,we have an ancestral land at our village of 3 Bigha.From last 2 years one man of my village courting that now this land is his property.He is showing some false agreement letter and saying that our Grand Father had signed this agreement.This is totally false,none of my family members signed any agreement and if there is 0.0001% chance that this agreement is signed then also how could our land become his property legally without registry.He frequently sends us notice from police station,IG office,CM office and our Tehsil by showing above agreement letter that we are forcefully farming this land and threatening him .We are not residing permanently at our village and his nuisance creats big trouble for us.Again and again we are sending replies to above notices.He is unemployed and residing permanently at village and it is very easy for him send us notices. Sir give your valuable guidence to me so that i can legally overcome from this troube.Advice me that how could i file a case of froud on him.Thanks a lot.