User registration problem

In order to allow people to register for forums on our site, I’m using the free version of the Profile Builder plugin. I’ve hidden the Register link in the Meta (Admin) menu using CSS (thanks to help from this forum), so the only place a user can register is from the page referenced above. Today I received notice of a suspicious registration–no first name or last name, just a long username of combined words, and an unusual email address: [email protected]. We are a small site of local naturalists and birdwatchers and this is not an expected registration.

Is this a known issue with Profile Builder, or is this a common issue with WordPress sites in general? Is there a better way to register users so that they can participate in the forums?

The page I need help with: [log in to see the link]