I gather you are sharing the same email account.
Go to Settings > Mail, Contacts, Calendars > tap your email account and turn off Note syncing. The notes folder in your shared email account will still have the notes in it, but the Notes app will not. Hold in mind that the notes you make will no longer be synced anywhere. Your only backup of them will be your iTunes backup, and I wouldn't rely on that for anything important.
If you have notes, they will disappear. To save them you can email them to yourself first.
If you have a second email account, it may also allow note syncing. In that case you could turn it on in that account.
Or better yet get an Evernote account. A much more versatile note taking setup. Your wife would probably like it too. Just get different accounts for it.