Can I cancel a push notification after it is sent?

Once a notification has been handed over to Apple Push Notification service (APNs) or Firebase Cloud Messaging (FCM), it cannot be recalled. When you trigger a send, Pushwoosh passes the content and the device tokens to those gateways and they own delivery from that point on. There is no way to pull back or delete a message that has already reached a gateway.

If a device is offline at that moment, the gateway stores the message and retries delivery when the device reconnects, according to its own internal logic.

What you can still stop

• Scheduled messages that have not been dispatched yet. While a message is in the Scheduled status you can cancel it in Campaigns → One-time messages (Message History) — open the actions menu (⋮) and choose Delete message — or via the API, and it will never be sent.
• Messages sent by the subscriber's device timezone. These are released timezone by timezone over a window of roughly 12 hours. If you cancel such a message mid-window, every timezone slot that has not been released yet is skipped: users in the earlier timezones already received the message, users in the later ones never will.

Replacing a notification that was already delivered

You cannot recall a delivered notification. On iOS you can overwrite one that is still sitting in the notification centre by sending a new message with the same apns_collapse_id. On Android the same trick does not work once the notification is on screen: android_collapse_key only makes FCM replace a message that has yet to be delivered to the device, for example while it is offline.