You can go to Tools -> Options in the main Thunderbird window, then click on Advanced. You should see a button labeled Config Editor... on the General tab. Click the button and type dummyEmail. Double-click the row that appears to make the value change to false. This won't completely disable it (it tries to add it always before doing something that would always break the address book), and you may have to reset your address book to remove existing dummy e-mail addresses, or update the contacts in Gmail.
To reset the address book go to the gContactSync menu in the Address Book window and click Preferences. Then go to the Accounts tab and click Reset All Synced ABs. Restart Thunderbird and synchronize again.
Please be careful when disabling this preference because this can cause problems in Thunderbird. If it doesn't work as expected please let me know because I haven't tested it.