Employment Law Questions? Ask an Employment Lawyer.
You have two potential available options here.
First, you have a defamation of character claim. You can sue that co-worker in state court for making false statements about you which have cause harm to your reputation. Some accusations (like accusing you of violating the law) permit an assumption of damage. As a precursor to this, you might consider having a local counsel send a "cease and desist" letter to just stop the defamation now, because typically when someone sees a letter threatening suit, they'll shut their mouths.
Second, this option depends on what you believe the basis for the harassment is. Not all harassment is illegal, in terms of employment law, meaning that not all forms of harassment require an employer response. If you can allege, legitimately, that this person is harassing you based on discrimination due to your race, religion, gender, age, disability or FMLA use, you can then make this a matter that the employer has to deal with...and if they don't, you then file a complaint with the EEOC. This option is only available though if you can point to one of those illegal motivations.