I have an Ubuntu Studio 18.04 machine that keeps getting pop-up notifications from a rather irritating source. I have tried 1) listing the URL in /etc/hosts, 2) using browser settings to block the URL, 3) using general settings to "block notifications". None of these actions have had any effect as far as I can tell. These interruptions include such messages as "Norton Utilities has expired", etc. (I have no expired Norton licenses on Windows machines.) Clearly someone or something is just being a pest, and all I want is to get the interruptions stopped. Comment is invited.