Young chinchillas usually do not understand they are hurting you ... you have to let them know that this action is not acceptable. A loud/sharp "Ouch" from who ever is being nipped and an immediate return to the cage will let her know this. Chinchillas love their free play time outside of the cage ... by loosing this each time, they learn not to nip. Usually. Chins are very smart and learn to associate things quickly.
But, at the same time, there may be something you are doing that it does not like, and that is it's way of asking you to quit. A hard nip is usually a warning that they will bite and bring blood if you do not stop what ever it is that is making her mad.
Chinchillas do not like wind/breath in their face ... also, you could pass on germs to a chin by blowing in it's face. With some chins, this may not make them stop, only more upset with you.
Jo Ann