**Try at your own risk**
Do you know your phone's MSL#?
If yes, then this is how I have always done it with my previous Samsung phones.
To delete your Sprint username, go to "Phone" and dial ##DATA# (##3282#)
"Data programming" screen should show up. Tap "Edit" and it should ask for your phone's MSL#. I'm currently using the MOAR Rom so for some reason it bypasses the MSL screen. Then tap "Data Profile" where you will be able to delete or edit your "Username"
For editing or deleting your MDN/MIN, go to "Phone" and dial ##MSL#. If your MSL is 123456 dial ##123456#
"MSL NAM info" should show up then tap "Edit" to edit or delete your numbers. I usually just put all zeros. Phone may restart after doing this.
Hope that works for you.