Why is Captcha used with Contact Form 7 not required?

Hi, I use “Really Simple CAPTCHA” with Contact Form 7, and can’t fix an issue: to make ‘captchar’ a required field. Here’s my CF7 code:

<p>[text* name placeholder "Name:"]</p>
<p>[email email placeholder "Email:"]</p>
<p>[text phone placeholder "Phone:"]</p>
<p>[text address placeholder "Address:"]</p>
<p>[text zip placeholder "ZIP:"]</p>
<p>[textarea details placeholder "Details"] </p>
<p>
	<span class="captcha-container">[captchac captcha-875]</span>
	[captchar captcha-949 placeholder "Code:"]
</p>
<p>[submit "Send"]</p>

Digged a bit on the web for some solutions, but couldn’t find anything viable nor any clue/answer on this forum. Can anybody help me, please? Thanks!

http://wordpress.org/plugins/really-simple-captcha/