Deleting a WordPress.com site permanently removes it and you’ll never be able to re-use its WordPress.com address (such as yourgroovysite.wordpress.com).
To delete your entire site and its contents, you can select your site in the app and go to the “Settings” section.
There, scroll to the bottom of the screen and tap “Delete Site.” You can follow the instructions on screen to delete it.
You can also learn more about deleting your site, and other options, here:https://en.support.wordpress.com/delete-site/
If it’s a self-hosted site you can only remove the site from the app by going to the Settings tab and clicking on the Remove Site option or by disconnecting the site from Jetpack, but this will not actually delete the site from the web. To do that you would need to contact your host.