Why would 'skulle' be wrong here?
That would mean that someone were supposed to stop them, but didn't.
Why does "noen bør slutter dem" not work?
"Slutter" is the present tense form, not the infinitive (slutte), but slutte is still marked wrong. Does anyone have any ideas why?
"Å slutte" is to stop in the sense of quitting; you can "slutte å røyke" (quit smoking), but you cannot "slutte" somebody.
I thought "Noen burde stoppe dem". Why is that wrong?
That's among the accepted answers.