How to remove a tenant if they demand for lifetime settlement!!

We made a house in the village of hooghly kolkata, because my grandfather want to make a tea stall. But my grandmother and grandfather thought of giving some part of the for rent for small business. We were not knowing that they have given for rent but ignored that because they will get some money for help. But what the tenant did, you know?? They made papers of Rent from local Lawyers and took sign of my Grandfather, and they mentioned in the Document that we have provided the rent for LIFETIME or 99 years and they can do any business without our permission. They were now demanding the legal document from my father as he is the owner but after seeing the situation my father decided not to give it for further rent. Room rent is just 500 per month and there are 4 of the tenant. As my Grandfather is not much literate and blindly signed the document and now the tenant are threatening my grandparent to give them the legal document. But my doesn't want to extend the rent of the house further that's why my father denied of taking the rent. But they are forcing to take rent and taking supports of local politicians and peoples. Now please provide me any solution to remove the tenants from the house.