The scheduled web push campaign can be suspended --just click on 3 dots for the particular scheduled campaign and click on suspend. However, already published one cannot be suspended.
When the Web Push campaign is published or sent, all Web Push Notifications are delivered to FCM/APNS gateways. The gateway starts delivering the notifications immediately and in case a user is unreachable, the notification remains as a part of the gateway till live time expires. There is no way to pull back or pause the notifications from the gateway. Hence, once when any notification is published, there is no way to suspend or pause the delivery.