How do you disable the Immobilizer Theft Deterrent System on a 2002 Volkswagen Cabrio as now the car will not start after attempting to use the valet key?

The immobilizer can not be disabled, You will need to go to the VW dealer and have the key re-programmed. If it is the correct key.
This problem can occur when the dealer replaces the ECU module - they sometimes forget to tell you that they will need ALL the keys to get them recognized as valid by the ECU.