You can also quickly delete all WordPress Users using MySQL or phpMyAdmin. This method is only recommended for more advanced users.
Login to the cPanel dashboard of your WordPress hosting account. Under the database section, click on phpMyAdmin.

Inside phpMyAdmin, you will need to locate your WordPress database. You will see a page like this showing all your WordPress database tables.

You can click on wp_user and then you see all users.

Click on username what you have to delete and then you see at bottom option delete .so click on the delete button.
PhpMyAdmin will now show you a warning asking if you really want to delete users.

That’s all, we hope this article helped you learn how to easily delete all users in WordPress.