Sending SMS messages is currently limited to use with Triggers though it is possible to programmatically send SMS using Custom Code.
There is currently no support for the ability to un-subscribe from SMS messages.
Programmatically Sending SMS
Sending an SMS message via PHP is very straight forward to do, all you need to do is first load the SMS model and then there is a function that allows you to send a message which will then interact with whatever SMS provider the platform is set to.
The following code wil send a single SMS message to the number provided.
$CI =& get_instance();