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:
You can just use the option
closeBoxURL : ""
http://google-maps-utility-library-v3.googlecode.com/svn/trunk/infobox/docs/reference.html
From Google Documentation
closeBoxURL | string |The URL of the image representing the close box. Note: The default is the URL for Google's standard close box. Set this property to "" if no close box is required.
Example
var infowindow = new google.maps.InfoWindow({
closeBoxURL: "",
closeBoxMargin : ""
});