Why can't they stop calling me?

Call from Naviss- a warranty extension company for your car with high costs.

They have been told 4 times I am not interested and keep calling

I receive calls from this number and company at least 5 times a day or more.  I have it blocked but when I call back it's a fax number with that long buzzing sound.  How do I get these annoying people to stop calling even when the number is blocked and they can't get through.  I want to call them and tell them I am going to report them but like I said I can't.

I am able to block this number, but still see that they've called - so far they have called 8 times!