remove custom field cache

I have written a script to automatically post youtube videos having custom field Name as ‘thumbnail’ and Value as ‘thumbnail url of youtube video’.

but the problem is all the custom field values are getting saved when I try to add a new post. I mean when I open ‘post-new.php’ all the custom field values that have been posted for the youtube videos are already there.

is there a way I can remove all the custom fields from ‘post-new.php’ and can somebody guide me to the right direction where i’m doing a mistake for which the custom field values are getting cached in the post-new.php?

I have WP Super Cache plugin installed.

this is how i am storing posts into the database.

$postid = wp_insert_post(array('post_title'=> addslashes($title),'post_content'=> addslashes($new_content),'post_category'=> array('12'),'post_status'=>'publish','tags_input' => addslashes($tags)));

  add_post_meta($postid, 'thumbnail', $img_url);