How to stop someone from using my stolen iPhone?

Follow this guidance→If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Beware of a common phishing scam: You may get a text or email from “Apple” or “Apple Security” (or sometimes “Police”) saying your phone has been found and to click on a link to see its location. This is NEVER from Apple or from law enforcement; it is from the thief or fence to steal your Apple ID and identity. Delete any such message without responding to it. Apple never gets involved in lost or stolen devices→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

Your carrier can blacklist your phone using them IMEI. This will block it in many (but not all) countries.

Assuming you enabled Find My when setting up the phone (it is enabled by default, but you can turn it off) go to https://icloud.com/find and put the phone in Lost Mode.

If Find My was enabled and the phone had a Passcode, no one will be able to use it. If neither was used, they can easily restore it and use it.

Apple doesn't get involved in lost or stolen iPhones. You can contact your carrier and ask them to Blacklist it, so it can't be activated on any networks.

Apple does not get involved with lost or stolen phones. Even if they did, the IMEI would not be used.

Please follow the instructions that you have already been provided.