I don't think any damage will result from just inserting a dummy plug into the KN7000 headphone socket, however, if you wish to be absolutely correct, you could make a dummy load to simulate headphones, by fitting two resistors of about 10 Ohms between the pins of the Jack Plug. One resistor between sleeve and tip and the other between sleeve and ring. Alternatively, you could just plug in a set of headphones but not use them !!

The method John mentioned to mute the internal speakers is :

Press Program Menus > Sound > OFF - (Exit x 3) this will mute the Internal Speakers and the Headphone output. A reminder that the 'Monitor is Off' will appear at the top of the 'Home' screen. This setting is not retained through a power down, so you will have to reset every time you switch on - not quite so convenient as the 'Jack Plug' method......

If you don't feel able to make the dummy jack plug, and still wish to use this method, let me know and I'll make one up for you.