I agree the translations they get from the dictionaries are horrible. Phrasal verbs, idioms, just don't work.
I'd recommend you tell them to use dictionaries when they are doing homework but not when the lesson is going on.
The main aim is to limit their attachment to the dictionary because it affects their ability to think or figure out simple meanings of words.
In my business English class where I teach adults, I always use this example: "The more a manager depends on his personal assistant, the less skillful and inexperienced he becomes."
This expression communicates the role of the dictionary on their learning process and they do get it because the expression reflects what goes on in their real lives.