Thinking about this again it would be very good if a moderator could simply shade in the parts at the end of a message that are a human readable signature and identify them to discourse as such. Discourse would then whizz through all old posts and hide any parts where the exact phrase/pattern of plain text/html/mark up was found.
A separate page that showed all these items could be created (perhaps listing how many times each had been removed as a visual check things were working) so admins could remove any (thus allowing redisplay) should the need arise. Imagine it as a global search and replace that ran (temporarily, so only affecting the displayed portion) replacing signatures with a clickable […] or similar.