If they are telling lies, you need to hire an employment attorney, pronto. And not only do they need to be telling the truth, they need to be able to document any negative comments made. That is why most businesses no longer give references of any kind. About the only thing a business can get away with as a negative comment is the generic, "they are not eligible for rehire". False, negative comments about you is actionable when you can prove it damages you economically, and keeping you from getting a position surely qualifies.
BTW, like everyone else, sure are curious as to what they are saying. I wouldn't post who they were, because that might come back to bite you. Good Luck.