One more thing to check: in the Legacy GV webapp (google.com/voice/b/0/redirection/voice) in the Groups tab, the Screening is On for anonymous callers in addition to the original place that I listed above?
Hopefully marking as spam will help too. One thing I like about Voip.ms vs. GV is that in the former, I can mass block an entire range of number using wildcards, which is rather satisfying especially since I setup the rules to ring as busy vs. just hanging up to keep their systems online a bit longer vs. freeing them to disrupt their next victim.