you can try using viz.mouseScale() and set the scale factor for the navigation to 0 and then switch it back to 1 when you want the simulation to move again.
For example:
Code:
def goStopTask():
viz.mouse.setScale(0,0)
yield viztask.waitTime( 2)
viz.mouse.setScale(1,1)
vizact.onkeydown( ' ', viztask.schedule, goStopTask )
when you hit the space bar, the program will scale the mouse input to 0 for 2 seconds, and then back to 1.
Best,
Masaki