How to remove someone else's Apple ID from an iPhone that was stolen from me

Half a year ago, an iPhone 6s plus was stolen for 128GB, it was not possible to find it today, the phone is blocked, there is someone else's account, not the one that was tied, most likely the phone was flashed, checked the IMEI completely matches the one on the box and the one in the phone, the question is how do I unlock my phone if I have the phone itself and all the documents from him in the flesh to the receipt, the person who stole the phone and used it is not known to me

[Re-Titled by Moderator]