You can usean external monitor with the iMac, but it will only mirror what is on the other display. I believe the output is VGA andonly supports 4:3 aspeect ratio with a resolution of 1024X768, the built-in dispay supports higher resolutions and a 16:10 aspect ratio (widescreen) but will match the resolution and ratio the external display. I don't believe there is a way to actually turn off the built-in display, though you should be able to turn off the backlight by reducing the brightness all the way. This will leave a faint image on the display (with out illumination) that can only be seen ifyou look closely in the right light.
Please tell us why you wouldlike to do this? Did the LCD in the iMac break, and you don't want to see the broken image alll the time? Or is it for a presentaion where you don't want the screen to be distracting peoplefrom the presentation?