Removes the "Gimme a site / Just a username" option from Wordpress multisite signups. http://git.io/NO6z0A

remove-blog-option.php

<?php

function remove_signup_option(){
  //Predefine what kind of registration is happening. wp-signup.php won't show the option if one is declared
  global $active_signup;
	$active_signup='blog';
}
add_action('signup_hidden_fields','remove_signup_option');

This can be placed in the primary theme's functions.php, within a plugin active on the primary site, or in a plugin in the mu-plugins folder

For quick implementation without coding, use these steps:

1. Download the PHP file by click clicking the raw icon above (see image) and chosing "Save link as"
2. Upload that file to the wp-content/mu-plugins folder