How do I remove and ban a hijacked subdomain of my site?

Question

Domain Name System

Hello! I recently moved my site to cloud hosting with Cpanel and Litespeed and discovered that someone created a subdomain with my domain, which is not in the panel and which I did not create!
I have a handicraft blog, and the subdomain is a pyrotechnics store, and all links lead to their main store, which is located on the same hosting with the same IP that I have. First, they (i.e. the owners of the pyrotechnics store) created a subdomain api.mydomen.ru. I contacted the hosting technical support and they replied that they did not know anything, did not understand, and did nothing. I also wrote to the registrar, with whom I have been renewing the domain for the 4th year now. They advised to change NS.
I connected to Cloudflare and changed NS, but the CNAME is registered with the hoster.
After this procedure, the api.mydomen.ru subdomain stopped working, I thought that the issue was resolved. But a little later they connected a new subdomain - autoconfig.mydomen.ru.
Please advise what should I do to completely disable subdomain hijacking? Will moving to another hosting help and is the hosting to blame?
This has never happened before with other hosts.

Answer 1

CNAME is registered in the dns zone, not the hoster.
Voice the domain. Well, or check which NS servers are listed at the domain registrar (whois command), which NS servers are listed in your zone. Check that there are no such entries in your zone. If there is, then someone other than you has access to zone management. If not, then subdomains should not work....