Is there a way to call a drupal ajax function on a multistep form without causing the form to resubmit?
The ajax call has a callback function that has a few commands including what it needs to render again. In this care i image it starts rendering the whole form again. You will see someting like this in the callback. 

$commands[] = ajax_command_replace('#form', drupal_render($form));


And you want something like this

$commands[] = ajax_command_replace('#form .ajax-section', drupal_render($form['ajax section']));