From what I see, in v2 of GMaps API there was a property \"buttons\" of the InfoWindow object that one could define in a way that given InfoWindow has no close button:
closeBoxURL: ""
as stated before doesn't apply for InfoWIndow. This is an option on the InfoBox.
You may for example use this CSS workaround to remove the X, and the surrounding clickable button:
.gm-style-iw + div {
display: none;
}
And as davidkonrad said. This is a workaround on the code as it is today. It will likely be changed.