Stolen phone

my iphone was stolen however i wasn't unable to use find my iphone due to the person clearing my data and flashing it, the phone went offline for a while an i just saw it available again. However, it isn't in my devices it is just showing up on my other phone to remove from account with the person's number and stuff but i can't shut it down using find my due to it not being in my devices. This person was very dishonest PLEASE HELP!!!