Same problem here. Here's 2 workarounds to try:
1. While you're accidently ringing your boss and can't hang up, go to Menu>More>Exit in Go Contacts Ex. Then the native dialer comes up and you can hit close on the number pad to get back to screen with red button. (Too much clicking for this workaround to be a solution)
2. Or if you mistakedly dialed your ex again then the Power Button can be set to hang up. Settings>Accesibility>Power Button Ends Call allows you hang up with the power button (this is a solution as long as you don't press the power button during calls, like when you start a BT Headset Call and turn your screen off before putting the phone back in you pocket)
I uninstalled and went back to the factory dialer and contacts, which isn't that bad anyway. The only things I miss about GO Contacts EX is that it had the alphabet down the right side in contacts, and the recent calls mixed in the dialer screen, and swipe left and right between screens.