I've looked at suggestions on StackExchange and made some changes to files in /etc but after each login or reboot it is up to 100 % again in pavucontrol. On this computer I hardly ever use the speakers, and I want the microphone muted, always, so I guess I could even block list a few kernel modules for sound (which ones ?). But why can a desktop user not easily turn volume to zero, forever ?

Edit : Thanks all. It was not StackExchange but on AskUbuntu from where I tried some suggestions. I also tried muting audio in Pavucontrol but that didn't help between logouts and logins. Now I have set the default audio device in Pavucontrol to Dummy, and that works. I can still play sound from a video in VLC, but the microphone keeps muted.