To block the IMEI you need to contact your carrier. Did you read the link I provided. So you know the person who stole your iPhone?
if you did enable find my iPhone then you can login to www.icloud.com from a computer browser and track and wipe and the likes if the iOS device is currently connected to the internet
So you did not have a passcode and did not turn on Find My iPhone?
How do you know they are using your iPhone with a different iCloud account?
If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support
Someone is using it, he knows the passcode. Is it possible to block the phone using IME?
How to block my lost iphone 6s using IME
