I beleive that the "White List" is not a list of apps but is actually the list of "Trusted Publishers" in your Certificate Store.
When you click "Always Load," Revit installs the publisher's certificate in the (probably user's) Certificate Store. To reset the Code Signing dialog, remove your certificate from the Store.
If the certificate was installed to the user's store, you will probably be able to do this from Internet Explorer > Tools > Internet Options > Content > Cetificates > Trusted Publishers. Select your certificate and click "Remove."
If the certificate was installed to the Local Machine (as admin), I do it this way, from a command prompt as admin:
Display Certificates (local machine):
C:>certutil -store TrustedPublisher
This will display a serial number. Find it and add it in:
Remove Certificate (local machine):
C:>certutil.exe -delstore TrustedPublisher serial_number