You can't prevent people looking whether or not they are still in your list, however not many will. There is no such thing as a notification when deleted from contactlist for your contacts.
There is no other solution really, unless you would settle for creating a new group, placing all those in there and keep that group closed at all times (using sort by groups)...
She should be glad you still allow her to talk to you. You can alsways tell that you like to keep your contact list very clean and that you don't want all to show there (for yourself or for others who happen to look at your screen). There are also people who periodically clean out their contact list removing all they don't talk to frequently