1. The children were watching more and more television. Finally, their mother put her foot down and told them that from then on they could only watch one hour of television a day.
2. I know how much you want a motorcycle, but I just don’t think it’s safe. I’m going to have to put my foot down on this and tell you that you can’t have one.

Compare to: lay down the law; draw the line (at something); read (someone) the riot act. In this phrase ‘put’ means to exert control. The expression is often used to describe parents setting rules for their children. The expression is often used in reference to a request, which is refused, or some form of current (bad) behavior that is forbidden.