I've been trying to get rid of the annoying "message sent" notification for two days. I called Sprint and they couldn't help (spoke with four so-called tech support people. One said he'd never been asked how to turn off the notification.)
I've followed the instructions you've been good enough to provide EXACTLY (multiple times).
Unfortunately, there is no such option on my phone.
I grabbed my friend's Q (Verizon user)... lo and behold, his has the option to uncheck the box. I went screen by screen. I'm telling you, no luck. Any Sprint people solve this? Thanks.