This post goes over how to disable Firebase Hosting for a site.
Prerequisites
Install Firebase CLI:
npm install firebase-tools
Login to your Firebase account:
npx firebase login
Disable Hosting
Disable Firebase Hosting for <projectId> and <siteName>
npx firebase hosting:disable --project <projectId> --site <siteName>
Disabling hosting will make your site inaccessible.
Delete Site
Delete site <siteId>:
npx firebase hosting:sites:delete <siteId>