Making money for nothing

I have 5 different accounts with MTN and on all of them I keep getting competition invites which cost 1.50 per sms to partake in but i keep sending stop and then a new nr is used for next comp and i send stop again again again and again at a cost of 1.50 per stop sms. If millions of users go thru this exactly how much does MTN make just by irritating the snot out of their users. Surely something can be done to stop this kind of theft.