Exclude tags from tag cloud

Hello, I wanted to exclude some tags from showing in tag cloud but let them be visible in pages. I know I can do that with wp_tag_cloud(‘exclude=ID,ID,ID’); in wp-includes/widgets.php but in which place exacly I should write it? Or maybe it doesn’t matter?

I was searching for tag cloud plugin which would allow me to exclude tags in page level (not code) but those which I found wasn’t good enough. But when I was searching for more I found out that tag cloud in wordpress.com system have option to exclude tags by IDs in page level http://en.support.wordpress.com/widgets/tag-cloud-widget/ so I’m wondering why downloadable version of WordPress don’t have that field? Is there a plugin which would add that one little field to WordPress tag cloud or maybe someone can write it? (I mean it won’t create different tag cloud widget but only modify the one that WordPress gives with that field.)