I was thinking something along the lines of a temporary ban on MK. At some point in the (hopefully) near future, I would like TOs to start banning meta-knight for, say, 6 weeks. This will allow other character's metagames to develop, as well as hone individual player's skills, so that when meta-knight returns, people will be better prepared to fight him.
If this is ineffective, I would suggest a 30% handicap on Meta-Knight.
If MK, after all this, continues to win tournaments, I think it merits a real ban. I think that the 30% handicap is fair and balanced at the moment, and though a 6-week metagame ban to increase other char's viability may help, it may not. Besides, some TOs might ban MK anyway, but the 30% handicap may convince them otherwise. Of course, such a handicap is somewhat controversial, but I think that it is reasonable in this one, game-breaking, extreme circumstance.