I make a contact called "spam" or "zspam" to keep it at the bottom of my contact list
then I set the ringer to "no ring"
I add the annoying callers to it, and when they call again, the phone doesn't ring.
when it fills up with spam callers, I make another one, like "zspam2"
not a perfect solution, but it works for me.
I get calls from this number all the time.