What can be done regarding retaliation and wrongful termination?

A coworker of mine has been making defamatory statements about me for several months because I would not date her. I came into work and brought this to the attention of my supervisors, who contacted HR. I told them the names of many employees who could substantiate this and that it happened during work hours which is against the code of conduct of my workplace. HR intervened and the defamation continued, so I put in my 2 week notice of resignation. One week later, the HR contacted me to let me know that I was getting a serious warning for my behavior and that unless I work for an additional year without incident, I could never work for this company again, or any of its sister branches. Their claim is that I came in to the workplace at a time that wasn't during my shift and approached the employee directly, which made her uncomfortable. I did come in when I wasn't working because I work the night shift and nobody from administration is there during my shift, and I did approach the employee first because our code of conduct requires this. Other workers were there and have come forward to say that I remained professional when I approached her. Does this sound actionable based on retaliation? Additionally, one of this employee's best friends at work has now made a false accusation about a friend of mine who was one of the witnesses to these events, who had been supporting my complaint, saying that my friend threatened her life. My friend has now been terminated, although another employee who was there when this statement was allegedly made said that it was completely untrue. Furthermore, the witness heard that the HR rep who she was going to speak to (who also gave me the warning) was unprofessional and was not following the policies of our workplace, so she brought a tape recorder to the termination meeting to record everything. The HR rep said that she could not record the conversation. The witness stated that law allowed tape recording.