Firefox doesn't show the "yes", nor Acrobat X, but SumatraPDF does. I modified your definition as:

\definefield[test][check][cbox][][]

And now the "yes" doesn't appear. However, I don't know how this affects the functionality of the form.

--
----------------------------------------
Los fines no justifican los medios, porque la medida verdadera de nuestro carácter está dada por los medios que estamos dispuestos a utilizar, no por los fines que proclamamos.
----------------------------------------

“You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty. You want to grab a politician by the scruff of the neck and drag him a quarter of a million miles out and say, ‘Look at that, you son of a bitch.’” — Apollo 14 astronaut Edgar Mitchell