GMaps V3 InfoWindow – disable the close “x” button

You can also do it through the css.

.gm-style-iw + div {display: none;}

edit january 2019
as @antmeehan said in the comment,

Google have changed the HTML, and the close button is now a button element rather than a div

So the css code to hide the “x” button is now:

.gm-style-iw + button {display: none;}