What action can I take if a tenant doesn't pay rent?

Hi My mother's tenant is not paying rent for the last 6 months. he is evading our calls, blocked our numbers, when we go to the property, his father threatens and says that he does not know anything, but they are refusing to move out. they have locked the place. The tenant has not signed the rental agreement for the last year also, he has not renewed it. As we are women, my sister , my mother and myself, they are bullying us, taking a stand to not vacate. Now we heard that they are planning to sublet also. Please advise on what to do in such case.