There is no interface control to prevent the ALT tags from being used as image titles but you can disable them with a quick edit to the plugin's 'wp-juicebox.php' file.
Open the 'wp-juicebox/wp-juicebox.php' file in a plain text editor and comment out (or remove) lines 977 to 983 inclusive.
Just change:

$title_element = $dom_doc->createElement('title');
$image_title = $attachment->alttext;
$image_title = $this->line_break($image_title);
$image_title = $this->strip_control_characters($image_title);
$title_text = $dom_doc->createCDATASection($image_title);
$title_element->appendChild($title_text);
$image_element->appendChild($title_element);

... to:

// $title_element = $dom_doc->createElement('title');
// $image_title = $attachment->alttext;
// $image_title = $this->line_break($image_title);
// $image_title = $this->strip_control_characters($image_title);
// $title_text = $dom_doc->createCDATASection($image_title);
// $title_element->appendChild($title_text);
// $image_element->appendChild($title_element);

The leading // comments out a line and it will just be ignored rather than parsed and executed.

(The descriptions will still be used as the image captions.)

Please note that the line numbers above refer to the current version of WP-Juicebox (v1.5.1.1).