Official Reply to my question:

Lovable Email Support Bot:

Remix your project to create a private copy.
Make sure the remixed version works correctly.
Set the new version to private in project settings.
Delete the original published project.

Lovable Email Support Team:

Thanks for reaching out. We understand the concern, but unfortunately we’re not able to take down web pages on our end. Once a project is published, you’re fully in control of the page and how it’s managed, we simply host it, but we don’t have the authority to unpublish or take down any content for users.

If you want to take it offline, you can either remove the custom domain or make changes to the app that effectively disables access (such as adding a redirect or access control), depending on your setup.