A: If you are still considering a severance agreement, I strongly recommend you consult with an attorney first to review the agreement, give you feedback, and assist you in negotiating it.
A: If the Michael has been posting false FACTUAL statements about you or your company, despite his knowledge that they are false, and reputational harm could occur, you may have a claim against him for defamation of character. If, on the other hand, he has been posting negative OPINIONS about you or your company, that is unfortunately not illegal and therefore a cease and desist letter would have no "teeth."