Old Mutual is a spammer

I constantly receive spam sms's from this company. I reply "No' as directed and in order to opt out. I send this off and then block the number and delete the message. Lo and behold, a few days/weeks later I get another friggin' sms! I suspect they change the number that the sms originates from in order to circumvent their number being blocked and their offer being refused. I don't mind being spammed once and then having my opt-out request honoured but to deal with a company that refuses to stop and uses various mechanisms to circumvent rules and guidelines makes me incredibly angry. So tired of this $%^&*BS!!