You write your post as if a toddler picking her nose is some huge shocking notion and will harm your child irreparably. I'm kind of laughing a bit at how OMG SERIOUS!!!!!1111!! you are about it when she's only two years old.
She's not going to get worms, honey. As gross as it is, it really won't harm her. Yes it's a habit you want to break BUT when she does do it it's actually helping to strengthen her immune system.
She's much more likely to get worms from eating sand at the playground.
EDIT: Not to mention the fact that the bigger deal you make out of it, the more she's going to do it just because she's getting a reaction out of you... and because she's going to prove to you she can do it no matter what you say.