Turn it off then hold down the Volume Down button and turn the phone on. You should see a menu and towards the bottom of the list is Factory Reset or similar. You need to press the Volume Down button seven times to get to it. It's to ensure no-one resets the device accidentally.
The process takes a few minutes then the phone will be as it was when it left the factory. You will have lost all your personal data but they may be damaged anyway.
Before you do that, it would be wise to take the phone to the local Police and make a complaint. That behaviour is a crime and has to be stopped. If someone is publicly tried and convicted, it helps other people in future.