Live and learn. I come to find out that "Report" does not send an email, but rather puts and entry into the Report module, accessed via the admin CP. Not nearly so convenient as an email, but at least now I know where to check.
Browse to "Settings> Account settings > Forum Signature" .
I guess I wasn't entirely clear. I - as admin - want to be able to edit a user's settings - any user - not my own (I knew how to do that already). Once in a while it becomes necessary immediately to change or remove questionable content in a user's signature and I don't want to have to wait for PMs or email to get them to do it.