C:\Users\YOUR_USER_NAME\AppData\Roaming\Tropico 3\ExSaves

Inside, you will find a lot of .sav files. These are the files that you can delete, effectively cancelling non only the save game, but also resetting the Island progress. This way, when you fire the game up, the islands will be untouched again, as they were the first time you played the game.
You will need to play the easiest maps before be allowed to play the hardest, unlocking them.