Thanks for the replies.
The thought of unplugging the keyboards had crossed my mind.
However a couple of the macs are mac book pros, which as I'm sure you're aware have a built in keyboard.
I've found a solution to the problem if anyone else is interested. There's a piece of software called plainview -
http://www.barbariangroup.com/software/plainview which can display fullscreen images with a kiosk mode that locks out all user input unless a specif key combination is entered.
Thanks for the thoughts.
Alex
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