My Sprint > My account > My preferences > Block voice. Below are the options that you'll see:
•Block all inbound calls
•Block all inbound and outbound calls
•Do not block any voice calls (all of your previously blocked voice calls will be archived for easy access later)
•Block all out-bound voice calls
•Block only the following phone numbers for inbound and outbound calls
You can block inbound and outbound calls for up to 50 phones numbers. Enter each 10 digit phone number below or quickly add numbers you've previously blocked by accessing your archive list (archive list numbers will be saved for 90 days).
For more detailed instructions, visit http://support.sprint.com/support/article/Block__restrict_or_allow_voice_calls_using_My_Sprint/case-fk158645-20101105-114511.