My iPhone was stolen

Did you have your phone protected with a passcode? Do you know the person? Could they know your passcode?

How do you know they are 'using your information'?

If you were using a passcode and the person doesn't know it.. they can't use your information.

You should review this link if you have not already

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

It isn't possible unless you have removed the device from your Apple ID Account

If you remove the device from your account after you erase it, Activation Lock will be turned off. This allows another person to turn on and use your device.