Is this defamation?

A jealous boyfriend of my girlfriend has told her untruthfully that I have a history of committing assault. Apart from beating him up (joke)is there anything I can do about this?
I want him to admit that he is lying. Is a lawyer able to at least write him a letter of warning just on my say so?

I would suggest that you not turn a relationship problem into a legal problem. You might just possibly have a slander claim, but a lawyer's letter may just aggravate matters, but hey, a lawyer can send such a letter, but it won't be free.

Jackson, if she loves you, she will believe you. If she doesn't, and she believes her ex-bf, then why waste your time with her?