[blind-eddie]

To a point yes. I have done the same thing.
But editing the config to make the guest account unalterable is only for the modcp or acp, not the front end, hence why the guest account can change the password.
That account would have to be stripped of more then just being able to edit the password.
I think I would remove the guest accounts access to the usercp all together or at least key items of it.
Can this be achieved from a plugin?

I am not trying to hijack your thread Krusty, I am after the same idea as you.