Multiple yahoo.com subscribers

Hi – I am new to WordPress and still getting my head around it. I have nearly completed a website that includes a blog page. I have had no subscribers until yesterday and now multiple subscribers – 20 in fact and more coming in my the hour – ALL from the same domain yahoo.com. No comments have been posted and although some of the user names seem to be real first and last names with numbers after them, there is no correspondence between the usernames and the email addresses, which also seem to include first and last names (plus numbers or other characters).

Unlike my Contact page, I see that to post a comment there is no CAPTCHA code required and I wonder if these ‘sign ups’ are automated and just designed to annoy me. I am using the WordPress Popular Posts plugin but I am not sure how much this plugin controls the form. Is there a plugin that enables me to use a better comment form that might also include a CAPTCHA code?

Website is here.

The comment form is at the bottom of this post (for example).

Thank you.