See this stackoverflow question: https://stackoverflow.com/questions/5825134/clearing-the-badge-when-received-push-notification
I have not played with the recent versions of the pushnotifications, but in the old one you could set the number to 0. But indeed you did need to send this notification out. As I recall the enduser does not gets notified. It will only notice that the badge is gone when set to 0.
Regards,
Ronald