Do not plan adventures!
Dungeon World does not lend itself to preplanned adventures. Any plans you make are bound to be thwarted, ignored or decimated by the players. If you have a climactic battle with a big boss in mind, forget it.
Instead, prepare your fronts and react to what the players do. Remember your agenda: Play to find out what happens.
Remember that in DW, you aren't supposed to write an adventure. The adventure emerges through play, and your job as GM is to play your fronts so that your players can find their own adventure.
That put aside, if your players somehow manage to remove a big big bad from your front's roster, then they did it. Play your monsters tougher next timePlay your monsters tougher next time.
But when a big bad disappears, there are repercussions. There will probably be some sort of power vacuum and the scales that balance Dungeon World will be disturbed. Maybe an up and coming even worse bad will up their game to take over, or hordes of minions will be let loose, rampaging around. Who knows. Let your imagination run wild.