Not as far as I know. In fact, the system prides itself on allowing you to modify everything in it.

One striking demo I saw once a few years ago (so details are a bit hazy) involved the become: message. This is a special operation for swapping references around. "X become: Y" has the effect of everything that previously referred to X to now refer to Y, and everything that previously referred to Y to refer to X.

So the demo was to execute something like:

    White become: Black.

which immediately switched all white pixels to black and vice versa.

I think the person giving the demo even demonstrated

    True become: False.

though that made the VM unusable pretty much immediately IIRC.