Sometimes, simple things work.
I few weeks ago, I complained about this to a friend of mine.
He said, "Answer, but don't say anything. The caller will think they've reached a dead line, and sometimes won't call back".
It doesn't work 100% of the time, but it does work.
The ones that bother me are to local ones that call and only change the number by a digit, after I block the 1st, 2nd,3rd, etc...