Two possible scenarios:
1. You can setup overage protection, where you put a credit card on file and in the instance that you go over your monthly bandwidth or Event Plan bandwidth, it will rebuy automatically to keep you going.
Check out our Overage Protection walkthrough for more details.
2. You can also just let the bandwidth run out, although this will cause your streams to shut off when you burn through the bandwidth on your account.
Please note that their is a two hour delay on analytic reporting, so it might take awhile for the system to detect you are out of bandwidth. Your stream will not shut off, though, until the detection is made. This means in some scenarios you might find yourself in the negative if enough streaming happens within a two hour window.