Go to sprint.com and you can block numbers under My Preferences or you can call Sprint Customer Care and they will block the number.
Blackberry phones have a call blocking app in App World as does Android in the Market. The Android ones seem to generate a fair number of calls to tech support, however.
If you have a blackberry you can go to http://appworld.blackberry.com/webstore. If you do a search for "phone number block", a bunch of apps will come up. Make sure you read the entire description and reviews. From my understanding, two of the best phone number blocking apps out there are "You Got Blocked-Call Blocker" (I believe this app blocks all 800 series numbers and all private numbers) and "Call Blocker Professional". Both apps cost $2.99 and are well worth it.
If you have an android device you can go to market and search for "block private calls" and there are several application that are free and some that cost.