I can live with that, thanks.
This turned out to be a little less than straightforward, so for people like me who might be interested in this, the procedure (specific to Mac OS 10.13.6) is:
1) Make sure you quit the game by pressing Command-Q on your keyboard or going to Menu Bar > Beyond the Chiron Gate > Quit. (Clicking the "Quit" button in the game window doesn't suffice; the window disappears, but the application is still running.)
2) Drag the following folders to the Trash:
[name of the startup disk]/Users/[name of your account on your Mac]/Library/"Application Support"/"Beyond the Chiron Gate"
[name of the startup disk]/Users/[name of your account on your Mac]/Library/"Saved Application State"/"com.electron.beyond-the-chiron-gate.savedState"
3) Empty the Trash.