General synonyms:
* Slander
* Libel
* Malign
* Disparage
* Vilify
* Denigrate
* Smear
* Blacken the name of
* Besmirch
Stronger synonyms:
* Incriminate
* Traduce
* Calumniate
More specific synonyms:
* Slander: Spoken defamation
* Libel: Written defamation
* Malign: To speak evil of someone
* Disparage: To belittle or deprecate someone
* Vilify: To make someone seem evil
* Denigrate: To attack someone's reputation
* Smear: To spread damaging rumors about someone
The best synonym will depend on the context of your writing.