I don’t believe there are any settings that let you do this. You could however patch the NotificationsPreview.qml file found in /usr/share/lipstick-jolla-home-qt5/notifications/ folder.
At the top of the file you should find the line:
property bool showNotification: notification != null && (notification.previewBody || notification.previewSummary)
Comment out that line and add this instead:
property bool showNotification: false
That will stop all pop up notifications. This comes with the usual warnings about backing up, being careful when changing system files and reversing the patch before updating.