block stolen iPhone

You need to follow a few simple steps described in If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Notes:

if you think they are using your iPhone they might have the passcode for the stolen device. in this case YOU MUST go online immediately and change the password for your Apple ID.

Next priority is to set Lost or Stolen mode in Find My.

Next task is to inform your cellular service provider, and have the SIM blocked so they can't move it to another phone and continue using it.

Finally, report the theft to Law Enforcement so you have a reference number for your insurance claim.

DO NOT remove or delete the device from your Apple ID. While it is held attached to your ID it is protected by Activation Lock. Removing it will make it available to be re-used by the thief.

Edit: If the thief contacts you to offer to return your device, do not fall into the trap and get scammed or robbed again. Feel free to sent police to meet him, but No Way should you go yourself

I'm assuming that you previously set up a passcode and enabled the Find My option on your phone.

No one is going to be able to guess your 4 or 6 digit passcode to get into the phone. The Find My feature allows you to locate the phone on a map, assuming the phone is "on" and it has an Internet connection where it is located.

Here's what you need to know:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Watch our for a message from "Apple Service" or "Apple Support" that says that your phone has been found. This is a scam to try to trick you into revealing your Apple ID and Passcode.

Best to be realistic. The phone is gone. Protect yourself as much as possible.