ANSWER:

You should tell them about this at least once, but after or before the actual khutbah begins.
You must not say this in the khutbah as this is also not permissible.
Once you have told them, your job is done. 

I have seen bunch of people talk during quthba. We are not supposed to talk or say word during quthba. What I believe is that our prophet Muhammed (pbuh) didn’t like while someone talked during quthba and he used to even get angry. What kind of action needs to be done on those who talks during quthba time? Sometimes we try em not to talk, but they start again.