The US can deprive foreign F-35s of combat capability without an "emergency switch"

 You don't need an emergency switch to stop the exported F-35.

The United States does not necessarily have to rely on an emergency switch to quickly undermine and soon completely deprive a foreign country of the ability to use its F-35 fighter jets.

In response to recent European media reports, Lockheed Martin and several governments have opposed the idea that the F-35 Joint Strike Fighters have any kind of hidden "switch" that the US authorities could use to remotely disable the aircraft. 

The harsh reality is that a special switch is not needed to prevent foreign F-35s from doing what they were designed to do. Simply ending aircraft support (including software) will give the same result, although perhaps not instantly, but soon enough.