You can not better someone that doesn't want to change
You know the saying. He fooled you in the past. Back then, they just did somehow cheat. They were banned. You gave them a second chance. Water under the bridge.
But instead of bettering themselves, they stepped up their game, going so far as to program manipulation software. They clearly never intended to change their cheating way but instead perfected it.
That behavior is unacceptable, especially in online play!
You have taken the appropriate step to cut contact and ban them from your table. It seems like you had been that ex-players means to feel themselves better by winning. To get his fix of winning they cheated and abused you. You as the gaming group. But also you as the GM. And you as a person.
Cutting them out of your life is the only way to not only protect yourself and your group from that abusive relationship. That player made your group his fix of winning. You are not someone's fix, you are a person with feelings and problems and likes. You don't have to cater to someone to give them their dose.
The troubles of chat-play.
Bots are magic? Not so much.
I have played WoD for more than a decade in IRC solutions. Back then, bots were rare. So we had to write our own dice-bot back then. Or rather one of the 1st generation players did write it. But the code of the bot was made open-source and read by the GM out of necessity. Even I had reviewed for secret functions and then tested vigorously, even as the bot was established for some two years when I joined. Literally, millions of rolls would be thrown and got quite close to the expected value. Which lead to the bot at times feeling a little too perfect, but we all could verify that the code was fair. And indeed it was - we had our fair share of good and bad rolls, and it worked well over the 6ish years that bot was used.
When we swapped to a different chat system, we took 3 different dice bots and tested them, deciding on one and a backup solution in case of bot-out. This time, none of the players had a stake in making that bot and we still can look into the source code. So we can trust that the bot is safe from player manipulation.
Dice-systems should be open-source and reviewable. If possible, use one not made by a player.
Communication is key
A gaming group lives on communication. I play the fourth-oldest player character in that WoD group and are pretty much a 3rd generation of players & characters - the older active PCs are 2nd generation characters. We had our fair share of joiners and leavers over the years. We had people on hiatus - and indeed, one character that is older than mine has been on extended hiatus for some years.
However, we also had our fair share of people that were let go: The player from the original crew that I replaced had messed up by goading another character into a duel to the death and then trying to blame the player for actually pulling through. Another player, that had for some time taken the role of GM, had introduced topics that the group consensus wasn't very comfortable with - and after the group aborted that plot, that player did not want to rejoin us back as a player.
But that latter episode taught us new methods to try and keep everybody on board with the plot, to make sure everybody enjoyed the game still. A literal decade of playing had turned us as people - and our playstyles - but good communication is the key to a successful long-running group. Without it, the group withers and dies. With it stifled, it festers miscommunication and inter-player resentment, which can seriously reduce the fun of every involved party.
Other players took extended hiatuses for work reasons, others had to skip days for health, and in the last year and a half, health issues have at times cut our sessions short at times. But that all is ok because we talk about it. We are there for one another, even outside of the 4-weekly sessions. When I lay felt physically ill some week ago, I got back kind words and a few cute pictures from them, and when I learn one of them is feeling down I do my dearest to give them m
Find a way to keep every player on the same page - and if there is a player that misuses their privileges... then that player is harming the group. Keep the group and everybody healthy - socially as well as mentally.