Defamation

Ok,,,I quit on my old manager, afer 2mos off,,, she called me for a job,, i took it back temporary of course,, hours of service completed. almost 3 rs later her employees were to go on strike,, she calls me i was to go and start working..it didnt happen, which is OK.
Now that I' trying to get back to the work field she is black-balling me. Employee Relations willing NOT to do nothing but told me to confront he. As is I can't get a job bcz of what she is saying to prospect employers, I asked her and she denied it yet she has told them nothing but negative things about me.. What can I do? Suggestions.