You can gently inform your previous employer that the negative references are harming you and that you will consider taking appropriate legal action involving claims for defamation and other torts. In that way, your previous employer will probably start doing what many employers do these days -- acknowledge your previous employment and provide your dates of employment and perhaps your compensation, but little other information.
Please note that I am a New York attorney and cannot advise you on the application of Ohio state law. You should thus consult with a local attorney.
Good luck to you.
Often one of the most difficult things in a case such as yours is proving exactly what your previous supervisor is telling prospective employers. Employees often believe (correctly) that their former employer is giving out bad references, but they cannot show proof that this is actually being done. If you can prove what your previous supervisor is saying, then you should contact a lawyer to determine if those comments are actionable. If what the supervisor is saying is false, you may have a claim for defamation. On the other hand, if the supervisor is just expressing an opinion (for instance, if he is telling people that he didn't think you were putting forth your best effort), then it may be difficult to bring a sucessful lawsuit against the supervisor or your former employer. Ask a lawyer whether the specific comments being made by your former supervisor can serve as the basis for any action.
Disclaimer: Please note that this answer does not constitute legal advice. It is merely intended to provide general information to aid the poster in finding answers to the problem posed. This answer does not create an attorney-client relationship. In most cases, it is best to contact an attorney directly to find answers to your problems.