Go to the Lists screen and tap the List Settings gear button next to the list that you want to protect with a passcode.
Scroll down in the List Settings screen and select the Passcode Lock option.
Tap Turn Passcode On and enter a 4 digit passcode.
Now whenever you open the list, you will need to enter the 4 digit passcode.
To disable the passcode, return to the Passcode Lock screen in List Settings. You will be asked to enter the passcode before it is disabled.
Note: AnyList Complete is required in order to use the passcode lock feature.