Well, found this web browser game when I was catching up on shippuuden: http://www.naruto-arena.com
.... it's a turn based game, where you form a team of 3 characters with 4 skills each (1 skill is always invulnerability for 1 turn). It seems to be quite skill based, they do update character balance, but I guess there are always imbalances. If somebody has been into this game for awhile and know what rules are used in normal tournaments and such, I guess they could say something about it.
You can unlock characters tho, so I guess there would be a point in only allowing the starting characters (22 of them, so there are some stuff to pick). I think you can check game logs, so posting that could work as "proof" for the match being played. And if somebody breaks the rules and refuses to forfeit/replay, then you could just take a screenshot or something (I think there's supposedly some ingame chat, doesn't seem to work in ladder matches tho).
It's hell of a fun game tho :p, but might just be because I'm a turn-based-game fan, but whatever ;o.