Yes. Require an email address for registration. Make sure the email address is unique (not already in your database). Send an email to the user asking for a reply with a randomly generated key in the reply, and use that as an authentication email so that you can verify that the registrant gave you a valid email address. Only allow the registration to complete when you receive the confirmation email along with the random key.

That's the same system you'll find in use across at least 75% of all websites that require registration. Now you just have to implement it for your site, and forget about IP's. About the only thing I would do with IP's is *maybe* put a 120 second block on a given IP once it's requested registration, so that a robot can't bang away at your site a hundred times a second.