What is the ruling on shunning one's father due to his harassment of others and his obscene language, even with his granddaughters?

You must enjoin your father to good and forbid him from evil with gentle words, and use available da'wah (Islamic propagation) methods such as audio recordings and pamphlets, or seek help from scholars. It is not permissible for you to shun or abandon him. Rather, you must honor him and maintain ties with him, even if he is a disbeliever, acting upon His (Allah's) saying, exalted be He: "But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them, but accompany them in [this] world with appropriate kindness."