The following steps are intended for the Kyocera DuraXT.
Press the OK/Menu button, this will display the main menu.
Select Settings, and then press OK.
Select Others, then press OK.
Select Roaming, then press OK.
Select Roaming Mode, then press OK.
(Note: This step will allow you to enable/disable roaming.)
Select Sprint® Only, to turn off roaming. Then press OK.