One way would be the following:
In your template index.php, locate the line where it says:
echo stripslashes($page['menu_title']);
and change it to
$text=stripslashes($page['menu_title']);
$a=strpos($text,"_");
if ($a>0)
$text=substr($text,0,$a);
echo $text;
This searches for underscores in your menu_title, prints out the part before the first occurrence of "_" or, if none found, the full string.