Taunt immediately shuts it down.
Encore forces them to use Cosmic Power over and over.
Haze and Clear Smog resets any boosts.
Roar, Whirlwind, and Dragon Tail forces a switch on the opponent's Pokemon.
If your team is weak to setup Pokemon, Ditto is a good option.
Something I often see suggested in the forums is to ban move a, ban move b, or ban Pokemon a. Imo, instead of using the forums to suggest to ban a move or Pokemon, why not ask for ways the Pokemon/move in question can be handled? Just as an example, Clefable can use a Cosmic Power set. A good Steel-type Pokemon with Taunt, like Heatran, can win 1v1 after a boost.
One last thing, if your opponent is playing like they're battling a boss, doesn't that make it easier for you to play around?
All in all, -1.