My iPhone 6 got stolen and I didn’t have the find my iPhone turned on

my iPhone 6 got stolen and I didn’t have the find my iPhone turned on in that phone and I would like to know how to disable it, i have the serial number and all the information. I know who stope it but the person doesn’t want to return it back & i need help to lock my phone & put it on lost mode & disable it.

[Re-Titled by Host]