Hi is it possible to shut down my iPhone permanently if stolen

No. There is no way to remotely shut down an iPhone.

You can lock and mark it as lost so it cannot be used, but you cannot shut it down.

click here ➜ Use Lost Mode in Find Devices on iCloud.com - Apple Support

Then that person can do anything they want with the phone, including changing passwords so you will be locked out of your own phone.

Call your carrier to disable cellular service on the phone so you don't have to keep paying for cellular service that you cannot use. But, the person will still be able to connect using Wi-Fi.

What if the person knows your passwords to everything