In order to hard reset a Samsung Z300, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Samsung Z300 device will be brought back to the default configuration of a brand new device.
1. For starters, press and hold the Power button until your phone turns on.
2. Tap the *2767*3855# code into your phone. Next, press Call button to confirm.
3. After a few minutes your phone should reset & reboot. Congrats.