Hi Lcchang
The first example really is expressing that you are serious or what you say should not be taken lightly.
When I say you can not watch televison before bed, I mean it.
I am sure when he said that he did not wnat to insult you, he did not mean it. (do not take it seriously).
And the second is more that you demand sth or wish some action to be taken.
You have watched too much televison before bed, you can not sleep. I am putting my foot down (stop).
I am sure when he said that he was insulting you, I would put you foot down. (take some action)