see your cforms Help page under "WP Comment Feature input fields":
To get it working:
1. Turn on the WP Comment feature for the given form. (Make sure it's the right one!), otherwise you won't see the above input fields in the [Field Type] select box.
2. Modify this form to include all the necessary (new) input fields, make them required or not, add regexp, anti SPAM fields or even custom err messages. All up to you. Or better yet, start with the built-in preset: "Advanced: WP comment…" form.
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You can however use CSS to set display:none for all fields you don't want anyone to see.