Hi! I've run into a problem on my site I don't know how to fix...

So I currently run a Sailor Moon forum and happened to come across a member from an old site I had. This particular member always gave me and other members weird vibes, and always seemed very elitest/entitled. We'll call them P. After being rude and disrespectful to a few of our members, my staff and I decided to talk to him about it. Instead of being understanding or saying that he would try to do better, P instead erupted into a flame of outrage and outright impertinent behavior. We ended up banning him from the site and the discord.

Fast forward three years, the same member tries to join the revamped site I currently have. My staff and I were weary, but decided to try and give him a second chance. Maybe they had changed.

However, I still felt uneasy about having them around and felt like they would just cause trouble again. They kind of just lurked in the discord for a month and half and didn't ever say much of anything. Never made an account, or never made an effort to communicate. So, I removed him from the discord.

The next thing I know, he is spamming the cbox and calling me 'toxic' and calling me and my staff team names. I banned the first few IPs, and banned him from the site, but I think he has an IPN (because the IPs keep changing), and he won't leave us alone. We already explained why we kicked them from the discord, and P's still masquerading as "his brother" and having an aggressive childish meltdown in the chatbox.  In one sense, it's kind of funny, because he's only  proving my hunch to be right -- and I should have just banned him to begin with.

I don't really know what to do, because I can only ban so many IPs, and he just keeps coming back and being a general nuisance in the site's public chat.

Should I just remove the CBOX and replace it with a discord widget or chatango?

I've never encountered this level of immaturity. Usually, people PM/DM the admin privately or they just leave.