If you have a Samsung account registered on the phone, you can log in to the Samsung account here to lock the phone: https://smartthingsfind.samsung.com/
If you do not have a Samsung account registered on the device, you will unfortunately not be able to do this. Then you can instead contact your operator to lock the IMEI number from the network.
Easiest then is to do a global IMEI block request so the phone becomes unusable by the person who stole it and ofcourse file a report to local police so if phone turns up at them then you will get it back.
