Disabling automatic teasers

Try using $post->post_excerpt instead:

// globalize $post, just in case
global $post;
// find out if the post has a defined excerpt
$data = $post->post_excerpt;
// If so, output something
if ($data) echo "<div class="excerpt">$data</div>";

This method will bypass the auto-excerpt generation inherent in get_the_excerpt().

EDIT

By popular demand, the same code, using has_excerpt():

// find out if the post has a defined excerpt
$data = ( has_excerpt() ? get_the_excerpt() : false );
// If so, output something
if ($data) echo "<div class="excerpt">$data</div>";