We may need more information - what sort of pop-ups, do they occur any time or when running particular apps, etc? If the suggestion below doesn't solve it, please post again with as much information as you can provide.
A generic suggestion would be to install an app called Addons Detector, scan the device, then look at the Addons list and check whether any apps are using "Push Notifications". If so, uninstalling these should stop your popups.