Oh thats very strange. If you hit ESC it should bring up the game menu, where you can quit! I can look into that in case theres something wrong with the inputs.
I see the problem. It's me. Despite hitting Z to start and getting the menu to start playing, I forgot that pressing Z to start actually means 'bring up the menu' when it was time to quit the game. Sorry for that but thank you for taking the interest in my issue!