I just noticed something: When I click on a user, the member card popup opens. In some cases I've got a "spam" option (next to edit and ban) and sometimes I do not.
Yeah, I do. I was wondering why I couldn't run the spam cleaner on some of my users but the link from @Brogan cleared that up.
Must be dependant on the message counter representing "0" then... The user has posted some messages, but the counter remained 0 because it was not being counted in those forums.