For a while it just feels like constant repetition, but honestly, they are taking it ALL in, so be consistent. Use your face and tone of voice. Keep it simple - a seven month old will be pretty close to understanding "no" by now! You can add other things like "no dont touch" or "no, ouch!", but it is the non verbal communication that means the most right now, so if it is scary, look scared, painful, fein pain, etc. And if she's smiling, she's either trying to get round you, to put that nice smile back on your face or cheer you up - only let that smile back on when she stops the bad behaviour, then whichever motivation it is, she will soon work out what she has to do to get you smiling again! And for everything you can, distract her, or you will hear "no" all day, which creates a pretty negative atmosphere. So also praise her loads when she does the right thing. And try making your language positive - babies seem to focus on the doing words first, so if you say "dont drop the cup" the will hear "drop", which ISNT what you want, so try "hold the cup tight". Dont worry, disciplining (training) isnt a big thing yet - wait another 12 months LOL! It's a slow learning curve for you both at 7 months and being smiled at is a bit nicer than the inevitable toddler tantrums she will try later. But by the time she gets THERE you will be ready for it.

We used distraction as our primary weapon and sometimes we would give in to the game. He's always trying to get in the bathroom, the garbage, under the oven (we fill with plastic containers, tupperwre, formula can lids, and mini bread pans). We would lay in between him and whereever he wanted to go, let him think he's almost there and grab him and make him start all over again. He enjoyes this.

We have a outlet on the wall that is in perfect plain sight and it took a long time to figure out how to get him to stop. Out of sight Out of mind. We put a square from a connect a mat flooring in between the end table and the wall. He will soon start pulling it out and getting at the plug but it doesn't happen as often.

Babies understand more then we think and the more they grow the more they know the words we teach them, at 7 months she knows what the word no means she is just pushing you to see what happens. my son is 8 months and is learning right from wrong now. i get the cheeky " You cant yell at me im cute" smile. If he goes near something he's not allowed to he get a firm NO, then is moved away from the object, if he goes back and touches he gets a tap on the hand and a firm NO and moved away. 

with the tap on his hand its not hard just a light tap but firm enough to teach NO. when i do it he looks at his hand then thinks about touching (most of the time he touches again)

it can get very annoying repeating your self over and over but they do learn and she will soon look at you as if to say "can i touch or is it a NONO"  and she will push your bounderies. my son waits till no one is watching him before he gets into mischief

Babies understand more than you think. It is true they don't understand it like we do though.

Even young babies understand the tone of voice.  If you use a firm voice when you growl, she will get that you aren't happy with her.  Say 'no' in a frim voice and ignore the behaviour.  one tiny thing though, a baby will try and smile to make you smile when you growl because they don't like the firm voice, so don't give in to that little game.

At that age you can't really do much. The only thing you can do is give them a firm "No" and take them away from what ever it is and distract them with something else. At this age they have no real understanding of right and wrong, but just a need to explore and learn through their senses (taste, touch, etc).It will take persistance and patience - but as she grows older, she will slowly learn that some things are "No".