Unless the radio host strays into providing factually false information, she is protected by the First Amendment. If the person is that affected, then he/she needs to stop listening to the broadcast.
One tactic that may help is sending a letter to the radio station. State that the broadcasts of the radio host are inflammatory and will be closely monitored in the future for defammatory language. If such language is noted then the defamed individual will hold the radio station jointly responsible for any legal actions. The station may talk to the host for fear of legal actions. It's not a guarantee but worth a try.

