First, of course, I blacklisted. Then, he starting both following me around on his main, and making alt after alt after alt. I took screenshots and reported every instance of in-game stalking. I took screenshots of out-of-game events (ie, he kept adding me on sock puppets on Discord and Steam).
Materia opened, so I moved to Materia. He left his main on Mateus, and continued his army of alts to harass me on Materia. I continued to take screenshots and to report the behaviour, quoting the ever-growing list of prior ticket numbers in my reports.
SE never suspended his account. They very clearly do not take this seriously. So I deleted my main and started over again.
He used a third-party tool to pull my IP address from Discord. He would create sock puppets, join servers that he knew I was in, and then spam my IP address and my (now changed) universal gamer tag. He would threaten to sexually assault me.
As he was now acting outside of the game, I no longer needed to rely on Square Enix. I put all of my screenshots onto a USB and took it to the local police station. The stalking began when he claimed that he was in love with me, so I had his full legal name, city of residence, and several unsolicited pictures from him to aid in my police report.
I never heard from him again. But I still don't post on my main on the forums. I have a new discord account and I don't add people I haven't met IRL. I don't add people to my in-game friends list. And, unfortunately, I don't participate in the action-adventure RP community any more.
Edit: And before anybody tells me I just should have ignored him to make him go away, I did. I never responded to any of it. Real stalkers do not get bored. They get dangerous.