No. She was lawfully ordered by a judge presiding over the case, and, as a member of the bar, she had no right to refuse that appointment. She did, in fact, request that the judge choose a different attorney, but that request was declined.
FactCheck.org - provides general background on case
Arkansas Judicial Rule 8.2 - Appointment of Counsel, which provides that:
Attorneys so appointed shall continue to represent the indigent accused until relieved for good cause or until substituted by other counsel.
Edit: Regarding penalties for noncompliance, refusing to represent the accused would be considered criminal contempt under state law, which provides in 16-10-108(a)(3) Arkansas Code that "willful disobedience of any process or order lawfully issued or made by it [the court]" is punishable as a Class C Misdemeanor, at least according to 2010 law. Also, professional sanctions might be involved, IANAL so no experience there.