Harassment from Debt Collector

I have received multiple calls a day for FNB debt although I have explicitly stated that they have the wrong person. I do not have a FNB account. Yet they are continuing to contact me relentlessly when I have demanded that they remove my number. I have blocked their number but they are continuing to phone from other numbers. This is harassment and a clear infringement of my rights. This must be stopped with immediate effect. How are companies allowed to operate in this manner??