Take the battery out. Or use samsung keis software on a pc
I just need to warn you that doing a hard reset will erase all your save files and application on your phone. It will be back to factory default settings.
To do a hard reset on your device :
1. Make sure that your phone is turned off
2. Press and hold the volume down + menu + power button
3. Wait for the samsung logo and release the buttons
4. You will see something loads on the screen and the recovery menu will come out
5. Use your sensor to navigate and select "wipe data / factory reset.
6. And press down on your sensor pad to confirm
7. Wait for the process to be completed and your done.