Try saying "Stop" instead? I currently have 3 full time 11-14 month olds plus my own 12 month old and they ALL know what "No" means and all smile and continue doing what they aren't supposed to be doing.
I have tried saying "Stop" and getting down to their level, stern voice, eye contact. Sometimes works, sometimes doesn't. In my experience though, I find when they are tired they are even MORE defiant.
I have my daycare on the main floor of my house so our playroom is gated off and I have a "confinement" playpen in the dining room in plain site. If after 3 strikes kiddos go in there for a few minutes then are welcome to return back to our group. Doesn't take long but they "get it".