Exclude category from drop down list form

try:

<?php
$cat_to_EXCLUDE = 50;
$cate_id = retrieve_cat_data(true);
$cate_name = retrieve_cat_data(false);
for ($i = 0; $i < count($cate_name); $i++ ) { 
?>
if ($cate_id[$i] != $cat_to_EXCLUDE){
<option value="<?php echo $cate_id[$i]; ?>"><?php echo $cate_name[$i]; ?>
</option><?php }} ?>

update:

if you search your theme’s functions.php file i bet you will find a function named retrieve_cat_data

anyway you can simple create a dropdown using wp_dropdown_categories
and you can exclude a category with its exclude argument :

<?php wp_dropdown_categories('exclude=50'); ?>