Put your foot down means to deal with someone in a harsh or strict way, or to use your authority to
to use your authority to stop something happening:
When she started borrowing my clothes without asking, I had to put my foot down.
When my son broke his curfew for the third time, I knew it was time to put my foot down.
You’ll just have to put your footdown and tell him he can’t stay out onschool nights.