Perhaps that was changed in the meantime, but nowdays I’d say !~= (which is documented in help page of note-viewer) seems to work just fine. So, if you check advanced mode, and in Filter textarea enter:
^
text !~= "StreetComplete"
you will get shown only notes which were not opened by StreetComplete user (i.e. in which first comment does not contain text “StreetComplete”).