Just to expand on the original post, maybe an idea could be a shared Alliance Inbox so that anyone outside of an alliance can contact an alliance admin. Then only friends or members of the same alliance can contact a player directly. This then stops any chance of sending different messages to admins, reduces message traffic and enables an easier audit trail should anyone become particular bothersome.
Any alliance that then like to message players before accepting into their alliance could use that shared mailbox, and all admins could monitor. I'd extend access to Co-Admins too (although this could be given to each alliance as an option)?