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I am sorry to learn of this situation.
Based on what you have posted, it is likely that you do have a viable claim for defamation (the defendant is stating something they either know to be false, or reasonably should have known to be false, to third parties). However, this is not a RICO matter.
Short of filing a lawsuit, you can try to mediate the dispute with them - contact your local bar association and request referrals to mediators, a third party neutral can often help you reach a mutually agreeable resolution. Use the bar association's referrals to contact a mediator or two, the mediator will then contact the other party to set up a mediation session, and you can go from there - hopefully resulting in a formal or written settlement agreement, and save yourself the time and expense of litigation.