Can we just switch off a malicious artificial intelligence? Let us assume we have a general AI that can improve itself and is at least as intelligent as humans.

It has wide access to technical systems including the internet, and it can communicate with humans.

The AI could become malicious. 

Can we just switch off a rouge AI?
Malware viruses are a very simple form of AI.  It is not difficult to conceive of a form of malware that A) can't be detected easily, B) is redundantly distributed across thousands of computers that occasionally connect to the internet, C) is capable of detecting some kinds of threats to itself and mutating to avoid the threats.

So, simply "turning off" a malicious AI will not always be possible.