No response to request to stop sms spam

I have several times responded as directed in Plumblink sms adverts by sending STOP if I do not wish to receive their messages, but the spam continues. I even called twice to branches and promises to stop was never fulfilled. Sending my complaint to their email on the website is of no use as \Delivery has failed to these recipients or groups:<br> <br> [Email Removed] [Email Removed]<br> The recipient's mailbox is full and can't accept messages now\"<br> "