Absa Rewards SMSs and I'm not a client

I keep receiving SMSs for Absa Rewards that are addressed to a Mr Kleyn. I'm not him and I have no idea why he gave them my number, but I've tried repeatedly to get them to stop sending me his stuff. The reason it can't be changed, is apparently because only Mr Kleyn can change his contact detail. I understand the principle, but that really doesn't work when the guy gave them the wrong info to begin with.