They have billing alerts ('beta') and used to offer a prepaid account type that they have discontinued for new customers (some may still have grandfathered accounts).
Closest thing now is the MSDN credit. It doesn't require a credit card and the account auto-suspends when you hit it. Problem with the MSDN credit is that it is for non-production only (and they reserve the right to kill anything they consider "production").
They should really offer prepaid again or bill caps. But Microsoft is too busy copying AWS to consider that they can do better than AWS.
