How can we stop this nusiance?

I am plagued by callers who don't leave a message or who turn out to be trying to sell something.

Every time I block their numbers it seems they just ring up from another.

Has anyone a good idea how to get rid of these menaces?

(I have an Iphone and my name is down on the Robinson list, but it seems sellers can disregard that if by accident or design you click on their online adverts!)