this kinda goes beyond my experience in that area, but theres probably something you can do with profiles. I am pretty sure you wont be able to outright refuse calls, but should at least be able to have unknown calls ring silently.
there may be a setting in the profiles to do this, or at the very least you could mark each entry in your phone book as "important" and make sure those are allowed to ring in the profile but make the default silent.