When that no longer stops them when they are about to get into potentially dangerous or very disruptive behavior, after the first "no!" you get up off your duff and physically remove the infant from wheresoever s/he has gotten into. No coo-cooing or laughing; stonefaced, non-positive re-direction.
When in the course of the next several months (baby now 14-18 months old) there is continuing problems/danger after you go through the drill, a sharp, stinging rap on the back of the hand accompanied by a "No!" will do the following:
--it will gain enduring respect for your "No!"
--it will possibly save your child's life or limb because now you can stom him/her from danger with a word from across the room
--it will make you all a happier family
--it will prevent the kiddo from becoming a brat
--you will only need to smack their hand rarely and essentially never when they get beyond three.
Amen. I have three children and do not beleive I have been required to raise a hand to them or utter the word "no" more than once in any specific circumstance after each one turn four years old.
My oldest is 15. She is taking college courses, in honor roll, and love gymnastics. My middle child is 11. He's in honor roll and loves karate, and my youngest just turned 5. She loves gymnastics and wants to marry Daddy when she gets older.
Spanking (not abuse) works.