I use Mr. Number on Android. This app has proven itself quite useful for actively blocking calls, but also blocking known robodialers, spammers, telemarketers, and other annoyances because it looks up the number before passing it through.
For iOS, you can make a contact and block it, but Apple's blocking mechanisms are rudimentary at best -- I shouldn't have to clutter my contacts with that. When another jailbreak comes around, I highly recommend iBlacklist which does a good job, especially with the pick up and hangup option (which gets the robodialers to get a person on the line, wasting their time.)