Simply tap the End button on the Call screen to end the call.
If you accessed other device functions while the call was in progress, there are several ways to return to the Call screen to end the call.
Tap the Home Key (in the Touch Keys bar), tap the Phone icon (in the QuickTap bar), then tap the End button on the Call screen.
Tap the Home Key (in the Touch Keys bar), tap the Call icon (at the upper-left corner of the Home screen), then tap the End button on the Call screen.
Drag the Status Bar down, then tap the End call button on the Status Bar.
You can also press the Power/Lock Key (on the back of the phone) to end the call, if you activated this feature in the Call settings menu.