How to say you don't want to renew friendship?

I think you need to tell them. Tell them what you've told us. Write them a letter maybe.

Don't ghost them though. That's infuriating and could possibly make him more determined

Just have your coat on ready if it's them at the door .
I do that if Jehova's Witnesses appear.

Stop replying, a little harsh but sometimes necessary, best wishes

We had someone years ago who was thick skinned and insistent. I just said “no, I really don’t want to thanks” and if pushed “why” said “sorry, I don’t want to talk about it”, or “I feel that it’s not for me” or similar. Something that can’t be argued with. And if challenged said “ I can’t help the way I feel, I’m sorry, the answer is still no”.
After maybe 3 or 4 attempts they gave up.
Once you give a reason, thick-skinned people will have a counter-one.

I understand your dilemma and personally I think the trick is simply to continue to be unavailable: block his number and if he contacts you in other ways simply say it's not a good time and that you will contact him when it's convenient.

Just keep saying it's not a good time. Be unavailable, leave texts, calls, emails unanswered.If your doorbell goes,have a peek, if it's them don't answer. If they turn up at your door say " oh, Im sorry Im just heading out to an appointment, meeting with ..... " . If it gets really bad tell them you are going on a long holiday.

Don't think there is such a thing mosaicwarts It will be kinder in there long term to be honest with the person concerned and say you have moved on now.
Cruel to be kind comes to mind.