Sorry, we do not currently have captcha on the registration page. The best thing to do is go into Administration > Site Settings > Security and enable the setting to require email confirmation for registration. This will send a link to the email address and the user must click the link before he will be able to login.
However, before enabling this you should make sure sending email is working by testing the contact form. If it can send email then it will work. But if email is not working the new users will never be able to complete the registration because they won't get the email with the link.
Hope it helps,
Joe