How to delete unsent message from google hangouts?
If a message was not sent yet, long tapping on the message reveals a menu. This menu has an option "do not send" (or similar). Clicking it, the message will be removed.

After the message was sent, there is no way to rollback - the remote side will be able to see it.

Text messages are sent practically on the spot, thus this option has only a little relevance. However, images send on google hangouts might have an enough long upload time to break their upload.

In the case of such accident, I think it is still the best to turn the phone off on the spot, then block its all possible network access (sim + wifi), and then solve the problem offline.