I can give you a query to identify the comments for a category. just replace 'my_category_slug' with your own category slug:
FROM wp_terms a, wp_term_taxonomy b, wp_term_relationships c
WHERE a.name = 'my_category_slug'
AND b.term_id = a.term_id
AND b.taxonomy = 'category'
AND c.term_taxonomy_id = b.term_taxonomy_id
however, using this to delete comments is DANGEROUS! It will not reset the comment_count in the related posts, and I don't know what effect this will have on the site.
IF YOU USE THIS TO DELETE COMMENTS, BACK UP YOUR DATABASE!