In your registry editor....Simply navigate to:
Under the registry key, create a new DWORD value with value name as SMSInboxThreadingDisabled and set it value data to 1. (then restart your phone)
Obviously...if you come to your senses eventually, you can change the value to 0 to revert back to threaded messages, or delete the dword entry entirely. (then restart your phone)
Click the credit link to also download a cab file to do this automatically. (Note that the .CAB file will also install the registry key to force enable the Sent SMS notification bubble.)
EDIT: cab file is attached.