This is exactly the number one reason to keep the game premium. I can't speak to the economics of it from the perspective of the devs, but requiring payment goes a long way to weeding out the clowns who want to be annoying and not take the game seriously.
Also, I find it bizarre that anybody has an issue with paying 2 bucks to try a game for a month. It's 2 dollars. I don't know about anyone else, but I have spent more than that on games that I've played for 2 hours and never opened again, and I'm pretty stingy about what I buy on the app store. Even if I choose not to continue my subscription beyond the first month, I've already gotten more than 2 bucks worth of amusement out of this game, which is more than I can say for a lot of garbage you can spend 2 bucks on (such as this revolting McDonald's milkshake I just bought on a whim).