Maybe your WP instance has the wp-cache plugin installed? That could lead to the results you're seeing. You can refresh the cache with the following URL:
YOURDOMAIN.NAME/wp-admin/options-general.php?page=wpsupercache&wp_delete_cache=1
Failing that, if you have MySQL access just do the following (this will remove the comments from the DB; not the markup inthe HTML/PHP templates to display them)
Code:
DELETE FROM wp_comments;
You can owe me a beer if I'm right!
