Ex Employer Threatening to Complain to My Current Employer

I have resigned from my previous company by serving complete notice period. At the time FnF I came to know that they are not paying Ex gratia so I wrote on the FnF form that I have not received my Ex gratia. After this the HR manager called me and said that you need to remove this line else we will not process your FnF. I recorded the call and forwarded to Labour Commissioner because the HR manager was replying to mail. after this they refuse to pay me anything. Then I came to know that there were some more employees who did not receive the Ex gratia so I created a group on Facebook named as "Ex gratia victims" to gather all such ex employees but I did not use company's name. Now I got a letter (without any letter no. that means it was no where recorded in their record that they have sent me such letter) from the HR manager threatening me that they will contact my current employer and inform them about previous instances of misconduct, unprofessional behaviour, low performance and negative attitude. these all are false. if I had done any of these during my employment, they should have issued me any warning letter or terminated me which they didn't. they don't have any supporting documents for these. If they do this can I sue them ? if yes then whome should I contact ? I have already given a written complaint to the Ast. Labour Comm. for my FnF.