hornplyr - I was totally inventing an awesome story in my head, about how the cheese and the chocolate ran away together on a thievery and murder spree, à la Bonnie and Clyde. We must stop the cheese and the chocolate.
But of course, Sitesurf's clear, logical answer prevails here. My figurative hat comes off to the DuoLingo hero in the figurative white hat!
Sorry Sitesurf, although I try to always try to take the temperature of a room before succumbing to humour- and even though I respect you immensely... This is a personal cheese and chocolate embargo we're talking about... Humour must find a way in here (no disrespect EVER intended). :-)
But I understood it more like a doctor's orders "Stop the cheese and the chocolate!", so I translated it as "Arrêtez le fromage et le chocolat.", but it was marked wrong. Without context, I don't see any way to get this right other than a guess (other than memorizing the Duolingo answer).
If the English sentence was "Give up the cheese and the chocolate", there wouldn't be so many comments laughing about this. "Stop the cheese and the chocolate" in English means that the cheese and the chocolate are running around doing things on their own that they shouldn't be doing. It's not like anyone is voluntarily eating the cheese and the chocolate, but more like the cheese and the chocolate are forcing their way down our throats.
It can be simple present or simple past (which is not taught by Duolingo):
conjugation verb "dire" in simple present: je dis, tu dis, il/elle/on dit, nous disons, vous dites, ils/elles disent.
conjugation verb "dire" in simple past: je dis, tu dis, il/elle/on dit, nous dîmes, vous dîtes, ils/elles dirent.
So what I gather from these comments is that "stop the cheese and chocolate" was accepted at one time, and then a bunch of people said it shouldn't be? It definitely should be. Read a women's magazine; they say "stop the sugar", stop the bread", "stop the margarine", etc, all the time (in the US, anyway).