What ways exist, if any, to block telemarketers and other unwanted callers?

My number is already on the do not call list, and has been for many years, but lately the sbags have been crawling out of the woodwork again. I even get calls from "private caller"s, which is fragrantly illegal. Is there some device or some such I can buy that will automatically hang-up on any unwanted calls, preferably without even ringing my phone?