I read Vince's post and while it made excellent points I'm left with the question of how to entrench the message of no means no in a child whose comprehension of language is weak to nil.
A child who is one year, 18 months old, has quite good comprehension of language. They know their names, they know the names of people who are close/important to them. They can't speak, but they know meanings. But communication is not just words, as you know. Tone of voice, facial expressions, laughter, all communicate meaning.
My own daughter knew what "no" meant, very early on and it wasn't necessary to use physical force to teach it. If she repeated behavior that we didn't want, she was removed from the situation or the object of her desire was removed. When she wanted something we didn't want her to have, she didn't get it. It only takes a few times from the message to kick in. The absolute worst thing a parent can do is say "no" and then give in when she kicks up a stink about it. That sends the totally wrong and conflicting message. They learn that if the are obnoxious enough, they get their way.
The second worse thing to do is always say "no". Say "yes" most of the time. Sometimes, as she got older, I'd say no just so her could make a good case for herself. I'd let her change my mind and debate the merits of what she wanted. It's about mutual respect. If you give respect, you'll get it back.
She wasn't perfect by any stretch. But she wasn't difficult either. Of course every one has a different temperament, but really, I think that a firm, not cruel hand very early on pays dividends as the child becomes more independent. It's just common sense really.