Defamation and Harassment After Leaving Company

I recently left employment with a company after lengthy employment. During the last 5 years with the company, I repeatedly reported a male co-worker and his wife for spreading lies about me, lying about my job performance, sexual harassment, incredibly inappropriate behavior, lying and attempting to report me to HR about things that either didn't happen or weren't true, and I have proof of all of this in writing where I reported this and more. I again reported it in my exit interview as a reason that I was leaving. During this entire time, nothing was done to this employee and instead, I was treated as overreactive and crazy. Since leaving, which I left under good standing, I found out from a mutual acquaintance that the co-worker has been telling people such things as I only quit because I was going to get fired (untrue) or that my company supplied apartment was disgusting and filthy (untrue and proven to be false). I reported it to the company's Human Resources Department and again, nothing was done. The employuer, without investigating as I requested and without taking any action, have closed the harassment and defamation complaint I opened with them. This employee has two previous harassment complaints against him within the last five years, one of which included the safety of a fellow employee that he locked in a room and wouldn't let out while yelling sexually profane things through a door. During the time that I worked there, I felt hunted, spied on, harassed and more. I'm not sure what to do! This breach of my privacy with untruths and harassment has to stop, and the employer won't do anything!