To keep your customers happy and to make sure that you can't be accused of text spamming them in any way, it's a good idea to have an opt out service that allows you customers to opt out from receiving further texts from you.
The opt out service works, using a keyword and a five digit short code. Customers who wish to unsubscribe from your list, simply text a unique keycode to 88802. Their mobile number is then captured by your account and added to your unsubscribe list.
Your account will then always check your unsubscribe list before sending out an sms campagn. This means that you cannot send an sms to an unsubscribed customer by mistake.
The code will already be on your account, if you go to message control then select the received messages tab you'll be able to see the opt out code from the drop down menu, the number for your customers to send the opt out code is 88802.
If you go to the send SMS screen and then where you'd type your message click on the 'Stop' button, this will give you an example of the text you can include within your message informing recipients of your opt out code.
Do let me know if you need anything else.