Open Compiz Settings Manager from Menu/Settings. Look for the Magnifier and open its settings by clicking on it. You will see the hotkey for start (and also for stop!): it is Super-M. So, you can kill Magnifier with this hotkey.
But if you untick the Magnifier, it will be disabled and never starts.
Otherwise, the Magnifier app is buggy: it starts unwanted triggered by other Compiz hotkeys.
At finishing moving a window starts magnifier; window decoration blinks
These are some of the reasons why I do not like Compiz.
Openbox-LXDE is wonderful, in my opinion it would be much more appropriate for Knoppix.