If you have a Duke Nukem Forever retail disc it is not neccessary for a large download to take place. The only download that would take place would be after the installation finishes, when any updates are released.
Open your Steam client and find the game in your Games library. Now right click on Duke Nukem Forever and select 'Delete Local Content'. After deleting the local content browse to C:\Program Files\Steam\SteamApps\common and delete the Duke Nukem Forever folder.
Once complete double check the games list in Steam to ensure that the game is not downloading. Now open the contents of the disc and launch the setup application to run the install.