How to deal with someone who is blackmailing and defaming me?

Some infamous local land grabbers are sending us whatsapp messages and calls and are threatning to defame us by making Posters with our pictures and defaming slogans and taking out possessions and also circulate such posters in social media and make false stories and use some local media houses owned by them to further promote these things in city. They want us to sell our land against an agreement which they have not even paid for. They have done similar defamation activities with other people in city earlier to pressurize them to do things they want. Can we file a blackmail/extortion/ defamation case. As proof we have received whatsapp messages from the main culprit showing sample of posters and voice note threatening and abusing us. How can we prevent such defamation activities.