What recourse do I have when my unstable ex from 10 years ago files a hr complaint against me at my job of 11 years claiming to be a fellow peer. He filed it anonymously but i was given the details of who it was after my

Divorced for 10 years. Ex attained free legal plan through an ex employer that he ported to keep. I have primary placement of kids and always have. For 4 years ex and his lawyer would send letters and drag me back and forth to court all in attempt to get more nights awarded ONLY TO USE the shared placement formula. He was awarded a Sunday overnights which was enough. I was promoted and make more $ and now he is starting this again since it doesnt cost him anything. Finally i told him 0$ and he gave up all time previously awarded except every other weekend. Now last week i get a complaint at work. My Hr team confirmed for me that it was my ex after confirming he was not an EE. I'm waiting to get something in writing however with a large corporation while working with HR complaints in my role, I know it is going to be tuff to get this. I'm in Wisconsin. Guy is nuts. What is next since he didnt get me fired, hire a hit man to kill me? Complain at husband's work now?