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