If your organization exceeds your monthly API call limit, Stream is alerted and a team member will send out an email notifying you of the exception and giving you the ability to upgrade your plan. If no action is taken by you to respond or upgrade your plan, the rate limiter will turn on to get your total number of calls under the limit for your plan.
Overage on updates adds overage fees to your charges for that month. If your organization is on the free plan and exceeds monthly updates, you will receive an email notifying you of the need to upgrade to a paid plan. If the organization isn't upgraded it will be suspended.