How can I prove that former co-workers are sending faxes and emails with false acquisitions about me to every potential employer

During my exit interview I reported on my supervisor and the Chief Engineer who have been fired from their jobs within a week of my departure. Now they have stepped to a new level and they have access to every company in IL since they are in the staffing industry. They are on a smear campaign and are faxing and email false acquisitions about me to every Sales opportunity in IL. It has been two years and I have lost three jobs after they contacted my employers. - Is this your question? Add additional information