Interesting. I'm working on a game that's strictly mouse/touch input. I don't use keyboard input for anything.
I just tried pressing the Enter key (one of the input actions mapped to "ui_accept") while the game is paused, and it unpaused it. So it must be default behavior.
To prevent this, maybe you could add your own code that handles a "ui_accept" action, and does nothing if the game is paused. I'll try that and see if it works, and post the result here.