If the number is showing up...call Sprint and see if they can do it for you. If they're blocking their number, they haven't figured that out yet ( I think ) and the last thing you could do is go into the digital lounge from your computer, click the free stuff, and download a silent ringer for the number you don't want calling...again, if they're blocking their number, you can't even do that...Good luck
If the number thats calling you shows up on the caller ID all you have to do is add it to your contact list. Then while viewing that number in your contact list hit menu then options and check off the box next to Incomming calls, Send directly to voice mail.
I also use a nice little app called CallFilter it will block(with options) any number and also block unavailable and private numbers as well. Its free in the Market.
RSPC44, This is the Instinct Forum, The Instinct does not have access to the Android Market. There is no app for the Instinct that blocks calls.
Here is a Silent Ringer you can place on the phone and assign to numbers you wish not to be bothered by.
Download it to your PC then use this uploader to send it to your phone.