Semck83 gave his word to play the game according to the rules of the pacifist variant organised in the forum advertising the game. If players had the ability to edit the in-game mechanism to reflect such variances, Semck83 would not have had the ability to move his units. Players do not have the ability to edit the in-game mechanism so the game's variant rules were based on honour and respecting what was agreed upon outside of the game.
So, hiding behind the notion that because it is a Diplomacy game the trivial issue of honour is irrelevant, for example, is a flawed one. Ditto the argument someone could make for there not being rules on this variant in the site rulebook - of course there won't be, it's a new variant that someone wanted to try out. Semck83 agreed to follow the rules of the new variant outside of the game, in this forum thread.
That he broke that, in my opinion, marks him out not only as an utterly dishonourable person but also as, what President Obama would term, a 'jackass', for ruining someone's best efforts to create a new and fun variant.
He hasn't broken a site rule so no intervention should be made against him - that's obvious. The issue is sportsmanship, honour, regard for fellow players, and respect of agreements made in the forum (which is not a part of any Diplomacy game) - this action shows that Semck83 lacks all of these qualities.