First of all, it should be warned that despite what you may think about the system, the authenticator itself is not 100% secure, this system is effectively a method of trying to make the authenticator less prone to keylogging attacks.
Disabling this feature is not recommended.
Disabling the feature requires some knowledge with the registry.
This has not been extensively tested, though i verified that it worked on 2 seperate computers.
Modifying the read/write permissions of the registry can be potentially dangerous, don't do it unless you know how to recover from such a situation.
(Tested with Windows 7 64-bit)
1. Open the registry editor (regedit)
2. Browse to HKEY_CURRENT_USER\Software\Blizzard Entertainment\Battle.net\
3. Rightclick the key named "Authenticator", click on permissions.
4. Click the user accounts who will be accessing WoW, (Do not touch SYSTEM, RESTRICTED or any Groups unless you know what you're doing)
5. Next to "Read" click the Deny checkbox and click apply/ok.
Every time you login you will now be prompted for the Authenticator.
Thanks to Deysi for the heads up on where to look :)