I have just created a couple of contacts called:
First Name -- DO NOT ANSWER 1
Last Name -- DUMB [***] CALLING
First Name -- DO NOT ANSWER 2
Last Name -- DUMB [***] CALLING
First Name -- DO NOT ANSWER 3
Last Name -- DUMB [***] CALLING
Every time someone like this keeps calling I had their number(s) to these contacts. If these contact names fill up to the limit of number...I will just add a #4. Then I set my phone to "automatically send to voice mail" and I never have to even be bothered with them.
This seems to work as eventually most have just stopped trying to get ahold of the non-existant person.