Besides some very hard, rude and some extremely racist messages, the rest should not be punished at all. Telling "F**k you" to someone is really far from being a good reason for a ban. This is a war game and message system is not only a way to communicate with friends, but even with enemies. Some times that's a way to provoke someone to attack you blindly and ambush him. System should allow minor offenses in the same way a duel is allowed in Hockey.
There are many provocateurs around who profit from this very static rule. They provoke with minor offenses and nobody reports them (because most of people think that is not worth for such little thing). Indeed they just shout a "f**k you" and they think is fair enough. Well, that's what he (provocateur) was waiting for...