To disable:
<div data-role="header" data-backbtn="false">
or
<div data-role="header" data-nobackbtn="true">
Regarding the setting, you can create your own return button, like this one, which mimics the behavior of the original, but allows you to control it:
<a href="#" data-rel="back" data-icon="arrow-l" data-theme="c" data-role="button">Back</a>
jQuery mobile Beta 1.