Hello! I was in a couple of threads about this a month ago. At the present time there is no way to block Anonymous/Restricted callers because you have to know ahead of time the callers number to block it. I sure wish they had a feature like this myself. As an alternative to be able to assign a "no ring" feature to such nagging calls would be great as well as blocking them from cluttering up your voice mail as well.
Now there is a back door work around to this that might help if the callers are pretty much the same people. You can use a program called "TrapCall" (you can Google it) can be used to reveal the number of those anonymous/restricted callers and then you can use that with Usage Controls right here to block those people. This works fine if you've got fairly consistent people doing this to you. However, if you are getting a lot of random callers then this isn't really the best solution.