Here is the TOS of the free trial:
http://www.windowsazure.com/en-us/offers/ms-azr-0044pIt clearly states that once you use all your credit
limit the service is stopped and you will have an
option to use pay-as-you-go plan after that.
Don't worry guys they can't charge you extra since its a free trial, its a 30-day trial and a 220$ credit. Trial ends with 30 days or after you spend all of the 220$ free credit whichever comes first. After that you can either leave and everything you have is terminated or continue by paying upfront for the amount of hours you want.
They'll never ever charge your credit card after using the trial credits as they need your consent to whether you would like to continue using their service.
Sure that solves the problem.