What Can I Do About False Accusations That Are Negatively Impacting My Career?

A former employee who is still with the company, but who no longer works for me directly, repeatedly falsely accuses me of discrimination. This began because I held the employee accountable, per company policy, for per performance and conduct issues. The employee resented being written up and, immediately accused me of racism. Her complaints were investigated each time and found to be unfounded.

Recently, while teaching a class, another employee kept falling asleep and I politely asked him to stand for a bit. He stood, then my former employee interrupted the class to ask me why he had been asked to stand. To help prevent further embarrassment for the employee and because doing so would have violated HR policy and disrupted the class, I declined to discuss it at that moment and continued to teach the class. She then went to HR with the employee (whom she supervises) and filed a complaint against me.

HR and my Sr Mgr immediately suspended me without first investigating the accusations (there were approximately 20 other employees present). They did not tell me what I have been accused of or what policy or standard of conduct I have violated. I am guessing she has again accused me of racism/discrimination, since that has been her pattern. What recourse do I have if I am fired? If I'm not fired, what recourse do I have to prevent this employee from continuing to slander and defame me?