How can I stop the flow if one of my human operators has already responded to the customer from the facebook portal?
There are two ways.
You can go into the Live Chat tab in Flow XO and start a live chat session with the user. This will stop bot interactions. You can also set up the Facebook Inbox as a Secondary app, and then simply move the conversation to the Inbox. Details about that can be found here: https://support.flowxo.com/article/237-facebook-handover-protocol