the nook has a way to stop purchases, there is a setting to require your password to make purchases i think its an on/off setting.
also it does require you to have an active CC on your account to make a purchase, so if you dont wanna allow it you can simply remove the credit card info.
but the setting has worked for me, my DD has had my old nook for a couple years now and has never been able to make a purchase on it.