Disable registration on certain condition

Since get_option('users_can_register') will return the setting value for the “Anyone can register” setting used for displaying the registration link, you could probably add a get_option filter overriding the value for the users_can_register key. In this case i even think the pre_option_%s filter is your best bet, since the actual value of the settings is irrelevant. Something like this might work:

add_filter('pre_option_users_can_register', 'wpse_87341_users_can_register');

function wpse_87341_users_can_register() {
    // Obviously here you'd populate $user_in_allowed_range
    // ...

    return intval($user_in_allowed_range); // We need to return an int since get_option does not like false return values
}