To fully delete all of your account data from Joomla.com, go to the profile menu and click on Change Details.
Click on Delete Account. Enter your password to confirm that you want all of your account data and Joomla.com website(s) deleted forever, and click on Delete. Be advised that this is the final step in the account deletion process.
You will get a confirmation that your account data has been deleted.
IMPORTANT: By deleting your entire account, all active Joomla.com websites related to it will be deleted too, and their subdomains will become immediately available for new registration. If you have any active subdomain redirects to an upgraded self-hosted website, they will also stop working with the deletion of your Joomla.com user.