Determines if the available space defined by the admin has been exceeded by the user
<?php wpmu_checkAvailableSpace(); ?>
If the amount of space remaining is less than or equal to 0 then it defines 'DISABLE_UPLOADS' to 'true' and sets an error message for printing in 'DISABLE_UPLOADS_MESSAGE'.
define( 'DISABLE_UPLOADS', true );
define( 'DISABLE_UPLOADS_MESSAGE', __('Sorry, you must delete files before you can upload any more.') );