Hi,

Do you have your old data on your sites?

If it's new site and you don't have any old data, you can use plugin Wordpress Reset to reset it to a fresh site.

If your site has old data, please do following steps:

a/ Install plugin Customize Reset

After that, go to Customize > click to button "Reset" at the top left of customize.

b/ Import demo data of any demo you want. It's way 2 in the guide here Way 2: Import Only customizer data

If you want to remove the demo posts/pages data only, you just need go to Posts > All Posts and Pages > All Pages > remove all posts/pages demo from demo data.

Best Regards,

PenciDesign