When I first started out, the question of metaing was also prevalent. Metaing is very different from say, multi-accounting, where with a bit of detective work, we can do a braddles on someone.
But metaing is impossible to detect if the 2 parties keep quiet about it. 2 different computers. maybe 2 different countries?
What if the person met him on the php diplomacy site, exchanged emails and say they will ally permanently in all their games, each one winning alternate games?
All punitive measures will just result in the 2 parties (or 3,4 parties) keeping quiet while they go about doing it. Without proof any judgment is rash, inciteful and has no merit. People will just game the system.
When the site was new, I always reminded everyone in my post that one of the best way to discourage meta-ing is to limit people to playing each other player one at a time. This will mean that I cannot play 2 games with the same player at the same time, and thus we cannot "share" victories, slowing down metaing by a huge factor