If it is a self-hosted WordPress, like we support here, then you need to delete the WordPress software and the database it is using, as well as the wp-content folder where images were uploaded.
You would do the file deletions using FTP or your host control panel File Manager, or if you installed WordPress using an installer from within your control panel, you would use it to delete as well (which would get the database also). The deletion of the database would be from within the control panel, if you didn’t use an installer.
If the site is not self-hosted WordPress, you are in the wrong forum, and would need to go to wordpress.com support since it’s different.
