No. S3, like most AWS services, has uncapped costs. If you experience higher than expected load, such as a DDoS attempt, you'll burn through the preallocated spend and you'll still get a bill afterward.

This doesn't appear to actually shut down the resources once the preallocated spend is exhausted. Its just a way to pay for bills preemptively instead of when you receive them. Its an accounting thing, not a new feature.