If you dont like giving her a smack then you could try what i do with my pony and that is smack my boot with a crop and say NO and raise your finger and point at them.
My pony knows now that having a finger pointed at him means no and responds to it - if he trys to nip i point my finger at him and say no and he stops... or sometimes he call's my bluff and pricks his ears and sniff's at my finger as if butter wouldnt melt!

gotta love 'em!