which cms? wordpress?
you could always strip html from comments
Code:
add_filter('comment_text', 'wp_filter_nohtml_kses');
add_filter('comment_text_rss', 'wp_filter_nohtml_kses');
add_filter('comment_excerpt', 'wp_filter_nohtml_kses');
or people that post urls , to make them just as text, not html so it wont index them.
Code:
remove_filter('comment_text', 'make_clickable', 9);