I've had good results using the guidelines found here on Twitter's page regarding impersonations - http://support.twitter.com/groups/33-report-a-violation/topics/122-reporting-violations/articles/18366-impersonation-policy
It's your best course of initial action. If that fails, you might want to speak to an attorney, depending on what Twitter's response actually is.
Disclaimer - the answering of this question does not create an attorney-client relationship. Further, this answer is for informational purposes only and is not legal advice.