Log into your Google account and find your way to your Google+
Next click on settings (gear icon), which take you into the accounts area
Click on security, then click edit on Authorizing applications and sites
Locate the Google+ hangout app you would like to remove (revoke access)
Click Revoke Access
The application should now be removed.
I wish this was a bit simpler, but it works for me; I hope this helps you out.
— Shortcut to Authorizing applications and sites —
https://accounts.google.com/IssuedAuthSubTokens?hl=en