It could be a run away application or just a bad day of a hacker making your life hard but it happens more often then it should. Applications log many messages, thousands or millions of messages, and it is going to take a moment to fix. Since Unified Logging’s billing model is based on number of messages it only makes sense to be able to disable the account and now you can.
On the Profile page there is an “Enable Toggle” button which will enable/disable the account for message receiving. If you disable the account and try to submit a test message on the Indexes page you will get a note saying your quota is met or the account in disabled.