In order to hard reset a Sony Ericsson K618, you should perform the following steps.

Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Sony Ericsson K618 device will be brought back to the default configuration of a brand new device.

1. Turn on the device.

2. Starting from the Home screen, go to Menu > Settings > General > Master Reset.
3. Press Left Softkey to confirm and follow the on-screen instructions.
4. Voila, done.