Well, I do have access to domain’s registrar and changing to Anastasia is not a problem.
Problem starts when some other hosts use Curt - they will get bogus response.
So, the question stands - how do I remove my domain from somebody’s else CF account without involving that somebody?
You need to change them to anastasia and boyd assuming those are the 2 you are being asked to set in your Cloudflare account.
The domain will become active in your account and the other set of nameservers won’t be called by resolvers. The domain will delete from the other account 7 days after the nameservers were changed.