Sprint monthly charges after cancellation

I cancelled service on my cell phone on 5/27/08...just four days passed the billing cycle. I was not under contract and had used zero minutes in the 4 days. Sprint cancelled my phone but billed me for the entire month. How can they NOT provide service and still charge. Since I wasn't under contract shouldn't it have been prorated? And if not...they should have told me this and I could have USED the phone for the entire month and called back to cancel on 6/19/08. Is this legal and can they DO this? I can't get anyone at Sprint to help above Customer Service who just tells me this is policy. When I get transferred to a manager I hold for over an hour and no one ever picks up. HELP!!!!