Employee is hurting company standards even after FNF

Dear Adviser, I am Gurbir Singh and an IT startup in Gurgaon. We had an employee in our company (Proprietorship) on Business Development Profile. He joined our company in 16th February and worked till March mid. We have already given his all pending dues via online bank transfer and he also wrote an email to the company against his full and final settlement with the company. Email copy written by our employee is attached at the end of this note. Now, employee has started hurting company standards by giving 1 start rate over Google, writing fraudulent notes against company over Google and other sites. Now he is threatening us and also calling us for more money and claiming that company has not paid him and he will file a case against company in labor court. We are a startup in the industry and need assistance of all of you respectable lawyers to play a safe side. Which legal action should we take against that employee? Please guide us with your expert advice. HERE IS AN EMAIL COPY - WRITTEN BY EMPLOYEE TO THE COMPANY REGARDING HIS FULL AND FINAL SETTLEMENT WITH THE COMPANY. "Dear Sir/Madam, This email is to confirm that I, employee name have received Rs. 20,580/- INR of including salary, leave encashment, Gratuity*, or other claims in relation with my employment with company. I am aware that I have signed and accepted on joining services at company name on February/16/2015. I solemnly declare that today onwards, I will abide by all the conditions of the said agreement. I have been paid through online bank transfer in my SBI account xxxxxxxxx99 as my pending salary from company. I understand fully about this Email and no conditions will be violated by me, which could hurt company standards or employees and I have nothing to claim against any pending dues. Regards employee"