Kids mimick people. Especially people who they are around them most. If they are consistently told the word "No", it is etched in their mind and can't help themselves. If they know they get a reaction from you, they will keep doing it.
You may need to try what has been effective with my son (now 3 years old son) along with my almost 6 year old daughter. This also works with other bad behavior.
I will get down to their level, hold my thumb and forefinger (firmly, but not hard) on there cheeks and put their face right into mine and say "That's ENOUGH" and once that is said, stare at them for three seconds (kids don't like to be stared at) and then let go. If they cry or show any remorse, then you got through to them. If they smile or it is ineffective, keep doing it until they do. It may sound cruel or mean, but it works.
Now most of the time, all I need to do is stare at them and say ENOUGH with a displeased look and they get my meaning.
In the meantime if they are doing something that they know they shouldn't be doing, ask questions like "Are we suppose to <<what they are doing>>. "Is that something <<behavior>> that we do?>>
If "No" needs to be said, try to replace it with "Stop" or "Quit"