You can 'always say no', sure, but that comes under completely ignoring the AI which means the AI can be of no benefit to humanity. You can't filter actions you want the AI to perform from actions you don't want the AI to perform, because you can't tell the difference.
The situation that springs to mind is that the AI, in doing what you believe to be helpful, sets up a situation in which it must be let out of the box. You are unable to see it coming almost by definition, because a super-intelligence just beats human intelligence very time.