In google glass you can click an Bluetooth item to disconnect it. But there is no way to click the icon on right side to unpair that device in it's subscreen.

In order to prevent paired devices from piling up, I wrote a small app in eclipse that allow you to unpair Bluetooth devices. If you have "No Launcher activity found!" error in eclipse, you can:
Go to Run > Run Configurations. Then, under Launch Action, change it from Launch Default Activity to Launch; and select the activity listed in the dropdown. Hit Apply and try running the APK again.

When the app is running on Google Glass. You will see a list of paired devices.

You can choose the device you want to unpair and click it. An asterisk will appear on it's right side.

Then click "Unpair" button, the selected device will disappear, indicating it is no longer paired.