I don't know how to block random pop ups on Android but if this has started recently it sounds as if it has been attached to another recent app download. Look back through your Downloads using Task Manager (all apps will have the .apk suffix so that you can install them). Sort them by Date and remove any installed since you started getting this pop up. One of them is causing it. If you haven't installed the free AVG antivirus for Android then install it now and run a scan.
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