Q: If someone does something nice for you but it turned out to be more of a hassle or you didn't appreciate the gesture, how do you politely tell them that you truly wish they hadn't? For example, someone gave you some chocolates or cupcakes when you are trying to lose weight. Or the baby sitter folded and put away all the clothes, but put everything in the wrong drawers, and you didn't ask them to fold the laundry in the first place.
A: Your examples well illustrate why there's no one answer: conditions can widely diverge.
As a general rule, respond in direct proportion to intimacy and repetition. The more of each, the more thorough your response needs to be.
That means the one-off gift from an acquaintance is easiest: At the front door, express genuine thanks for their thinking to bring you cupcakes — then at your first opportunity you head out the back door to locate the nearest food incinerator teenage boy and hand over the goods.
With your baby sitter example, repetition is possible and the relationship is right in your home — but you're the boss. So, provide clear encouragement and instructions. "I love that you took the initiative to do laundry, thank you — but I had trouble finding things, so next time please just fold and I'll put away." Or go the extra step: "I can show you where things go, too, because I appreciate the help."
And finally, for the loved one who makes a clear, precedent-setting gesture, like, "I made your favorite (elaborate dish here)," when (dish) jacks your flatulence risk to DEFCON 1, you just say what you need to say. "You're wonderful. You thought of me and I love you for it. Which is why it pains me to say I can't eat (dish) without consequences painful to us all."
Well — there is one more contingency, for the People Who Get Offended by Everything. With those, the response is always just, "Thank you!" (I can feel the tight smile as I type this.)