How to prevent meta data from being edited in a post?

I would recommend you to remove custom fields meta box from the post editing page. You can do it by using remove_meta_box() function.

add_action( 'admin_menu' , 'wpse8170_remove_post_custom_fields' );
function wpse8170_remove_post_custom_fields() {
    remove_meta_box( 'postcustom' , 'post' , 'normal' ); 
}