You must offer an opt-out in your message as it ensures that you do not contact anyone that doesn't want to be contacted again.
Here you have two options:
Sending from a long FireText Reply number
If you already have a reply number, you can simply ask them to reply STOP to your number.
e.g. Opt out? Reply STOP
Sending from a Brand (no replies)
If you don’t have a number, we can offer a personalised STOP keyword on our shared short-code; 82228. Simply get in touch and we'll sort this for you.
Here you can keep it short and sweet if you don’t want to use up too many characters.
e.g. Opt out? Text SBSTOP to 82228
You will need to put this caption in the body of your message somewhere.