Courier does not currently support notification deletion via an API. Our team is currently working on a Digests and Batching feature that would complement our Automations. Here you would be able to batch events into a single notification. Courier Automations has a cancel mechanism where you can cancel an automation run. Canceling an invoked send request in the middle of the message lifecycle would fall under a feature request.
One more thing I forgot to mention was that we have an archive endpoint that can be used for Courier in-app components like Courier Inbox. Not sure if this would apply to your particular use case, but please feel free to share what particular type of notifications you’re trying to send. Cheers!