How to disable CMS feature before I purchase a plan?

I have one-page portfolio site, but at some point I’ve enabled CMS feature, how to disable it? I don’t use CMS on this site. But when I hit “Upgrade to add a Custom Domain” it requires me to buy CMS Hosting instead of basic. How to remove this CMS feature?

Did you make the site from scratch or did you buy a template?

Templates sometimes include CMS content. You must go into the collections tab and delete all the collections.