I read once about babies and discipline, but remember very little about what I read.
(I start out too many sentences with "I read..." and end too many with shit I made up.)
What I do remember is that babies can learn the word "NO" pretty early on. They may not stop what they are doing, but they can understand your tone and the word associated with it. As long as you redirect your child after saying "NO" he/she can learn to listen to you.
(Okay, so I made up a little bit of the last sentence. I swear that is pretty close to what I read.)
Anyways, we have started trying this with Leila. She tends to want the things in our family room that she cannot have. Like the stereo, DVD player, cable box, and speakers. Instead of fencing off the area she can't play with (Duh! That would totally block the TV!) we have been telling her "NO" followed by redirection.
And what do you know...she gets it! If crying after we tell her "NO" (for the third time) tells you anything, then she's got it!
We pick our battles...a little dirt never hurt.