Go to the Digital Lounge on Sprint.com and select "Messaging". From here you can click "my block list" and add the number.
Effective *Monday August 30*, Sprint customer care can assist you with requesting incoming or outgoing call blocks. Up to 50 phone numbers can be assigned to the block list. This feature is separate from the current text-blocking capability. (http://support.sprint.com/support/videos/Services_video__Blocking_Text_Messages/case-wh164052-20100623-094808#__highlight&id16=block) Additional blocking features will be available in the near future including the ability to create "allow lists" and the ability to perform your own phone number blocks through our online self-serve tools. To block phone numbers, please contact your Sprint Care Rep.
Have your phone number added to the FTC's Do-not-call list to stop unsolicited call from getting to you: http://support.sprint.com/support/article/Block_telemarketing_calls/case-mi155155-20090701-154229#__highlight&id16=block+calls