As of Oct 14 "Inbound" call blocking is available.
Inbound call blocking is available through the Retail channel(visiting a Sprint store). This functionality only enables adding/removing phone numbers to a block list. It does not include blocking wild cards such as 8XX/9XX numbers, international calls or unlisted/anonymous callers.
More information on upcoming features is posted here; http://community.sprint.com/baw/message/222289#222289
Message was edited by: Levi4u
Also check out this blog post on using Sprint.com features to block unwanted calls or texts: