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You can certainly have a solicitor send a letter to him pointing out that he is defaming you and request that he desist in this behaviour. You can also threaten to sue him for defamation in this regard if he doesn't retract his defamation.
If He fails to desist and you have to issue proceedings against him, it is important to remember that even if you are successful in your defamation claim, It may be a Pyrrhic victory if he is not a financial mark