wp_delete_user with username

There is no way to do this with only one query. But to be honest – deleting user takes more than one query itself…

You shouldn’t use custom SQL for that, because there are many things you could break this way… Always use WP functions, if they exists (what if some plugin logs every deleted user, or what if some other action is needed, etc.)

You can use get_user_by to achieve that. Here’s the example:

$user = get_user_by( 'login', 'john' );
if ( $user ) { // get_user_by can return false, if no such user exists
    wp_delete_user( $user->ID );
}

The fields you can get user by are: ID | slug | email | login.