Ask

the friends plan not working, need to stop hanging out if possible

Don't just block her, make your stance crystal clear.

"We are not together. I am in another relationship. I understand you have feelings for me but it is not mutual. I feel that being friends is not going to work either. Goodbye." would be an example.

You gotta break up with her again. Don't be friends, she's not able to handle "friendship". Even if people are staying friends they should take a few months apart to let things settle.

Unfortunately, there are times where there’s no way to “be liked by everyone” and “have healthy boundaries”. Now is the time to be firm with boundaries.

Wishing you luck

“I’m not cheating. We broke up. Your next boyfriend will be glad that you think cheating is wrong, though.”

I hope you don’t have a pet rabbit…

I'd start by telling her.

Yeah, this unfortunately can only be addressed by fully blocking her in every way you can. Phone, social media, everything. Never agree to see/meet her in person even in public. If you run into her in person, say only that you're no longer in a relationship and then walk away and ignore her entirely.

Yea, this is why I think the "remain friends" think makes no sense.

You broke for reasons, seems those reasons would preclude wanting to be friends.