Unfortunately, unless you can state to HR that you believe the treatment against you in based on your race, religion, gender, age, disability or recent FMLA use, there is very little that you can do.
An employer doesn't actually have a legal obligation to deal with these sorts of issues...at least, nothing imposed by statute. If it is discrimination like I mentioned, then the employer can face legal action against them by the EEOC or Department of Labor, because there is an ongoing statutory requirement to handle complaints about discrimination.
However, without being able to complain about discrimination of the types I mentioned, the employer is free to handle these matters however they choose. The hope is that the employer would be considered about their legal liability if you were to be hurt (because they would be legally culpable) and, in their own self-interests, they'd take action. Regrettably, employers are not always smart and so they can't be compelled to take action.
All you can do, in terms of a positive right, is to quit and file for unemployment based on the fact that your employer never corrected the situation in the workplace after repeated complaints by you.