2 different scenarios from the AMP side:
1) using IGC = IAPs managed
IGC doesn't have a concept of monitor only mode, so you might have to disable IGC from the group basic settings page to revert control.
2) like previous replies showed -> on device's mgmt tab, set to monitor only mode.
if neither of the above paths work, you could try deleting the device from AMP, that should definitely revert it back to local GUI control.
Is there something you're trying to accomplish from the local IAP GUI that's not available from the AMP? If so, please let us know so that we can get it filed into a defect to be addressed.