The way you've expressed your concerns here is already very reasonable.
You could just tell them that you're dissatisfied with the game. They want to play a certain style of game, but you don't want to GM that type of game, so there's a clear mismatch. This does not need to involve any deception or proof or explanation. You are allowed to dislike things.
"I'm sorry, but I'm not able to run this game for you anymore."
That's all you have to say. It is short, direct, and polite. They may ask why, and you can choose how to respond, if at all.
If you tell them you're not thrilled with how they tackled certain subject matter, they may argue.
If you instead insist "I just can't run your game", it doesn't ...