Do you mean you cannot hit the back button and leave or you are stuck in conversation?
Not sure why you are bringing up the back button, I am talking about the ESC key. In almost every other window you can use the ESC key to leave that window and return to the galaxy map. Planet windows, tech window, govern window, but if you are talking to another race, you cannot ESC from that dialogue.
This has been mentioned several times since the game came out, no matter which window we are in we should be able to ESC to the galaxy map.
(Actually, the ESC key does not work in the Ideology window either. )