First of all I am glad you have decided to stop this because it is dangerous - human saliva contains bacteria that is extremely dangerous to birds, so this is more than just annoying, it is a health hazard.
As far as "no" what are you using to reinforce this word? It doesn't have meaning by itself, you have to teach the bird what it means. You can teach her the word "taco" means she must stop, and that will be just as effective because she doesn't speak English

We all have to learn that the word "no" means "stop, or something you do not like will happen." Now this is delicate place, because "punishing" a bird very often goes wrong. You should not hit your bird, shake her, scream at her, or anything like that. In fact, sometimes distracting her is enough, so try redirecting until she learns that she simply doesn't SUCCEED in opening your mouth. Make last time you gave in THE last time - do not do it again no matter what. Every time you give in you are TRAINING her to do this, so stay strong. If redirecting doesn't work, try taking something away - most likely yourself. Is she out most of the day anyway? If so, consider letting her spend more time in her cage. This way being with you is special (note:do not neglect her! I am simply suggesting you make your snuggle times a bit shorter for now, but make sure they are quality). When being with you is the only way she is out of her cage, things become a little clearer. So when she does this, say "no" in the tone you will use consistently, and redirect her to something else. If she returns, say "no" again, and put her back in her cage and leave the room for at least 5 minutes. Do this EVERY TIME. If you do not have time to do this, postpone getting her out of her cage until a time when you do. Substitute sitting by her cage to get your work done until you have time to train. Remember, every interaction you have with her (any animal, really) enforces behavior - make sure you only enforce the behavior you can live with; i.e. NOT putting her head in your mouth.