Go to Bluetooth settings and find your headset. Uncheck Phone to disable the headset from being used for voice calls.
When you have a Bluetooth headset connected to your Android device, making calls is allowed by default even if the phone is locked. To disable making calls when device is locked and connected to a BT headset, do as follows:
1. Open Phone application
2. Go to call settings, by pressing Menu Button and then selecting Settings
3. Tap Call Accessories
4. Tap Outgoing call conditions, and select Only when device unlocked