Sprint As You Go number: 855-319-4426
Bill can be easily waived by calling this number and simply ask to waive itThere is BLE"H 550 reply's to this post, please update the Wiki if you feel it's necessary
As I understand there is no reason to worry, if the account is completely canceled, it will automatically minus the difference at the next billing cycle, so for now you don't have to pay anything, just wait for a final bill to arrive
Finding a Sprint rep to cancel instantly and have the line prorated may take a lot of time. I was on the phone for 2 hours with reps who all said it would cancel at the end of the billing cycle and its against Sprint policy to cancel instantly and prorate. I finally git a rep who was glad to do it, and my line got cancelled and prorated.
You may also be successful in getting your account balance lowered by asking for a credit for the amount of time not used (it takes up to 48hrs to post)...
Make sure when you log onto your account your device is not there otherwise you didn't cancel
See this post to turn your phone into a tomtom GPS!