I am trying to find out why I am getting three of the same

I am trying to find out why I am getting three of the same ip addresses in a row using different usernames registering with the site social network http://www.fluffey.com. This is becoming a real frustration as some of these members are on the CBL blacklist. Know as spam bots. I have received 398 registered users in one week that have managed to click on the email activation link. I am going to have to find a way to allow only a limit per ip address to register into site. Can anyone help me also is there any form of captcha that like google captcha I can use for this may help also. I am going to have to look further it seems I am getting the same email address with different usernames is there a way to limit one email address per registration.
The pic is from Bermuda caves!

Regards Vinny