Mark CMG
Creative Mountain Games
It sounds like you might want to adapt the campaign a bit. If the group lived near a swamp and was breeding frogs to deal with an insect problem, would you lower the number of insects and eventually bring more predators into the area that would eat frogs or would you just let them know it worked and move on? You could have a large number of NPC arcanists using the spell, also, and have a large number of anti-MM devices start appearing on the market/scene. Of course, some players feel campaigns are more fun when the players are always having to react rather than the DM reacting to the players.. There will come a point where MM just isn't powerful enough to be used extensively anymore (becoming a waste of a round, in some battles), so you can probably just ignore it. Only you can determine how your group might feel about those possibilities, so good luck. 
