Ask HN: How do I stop spam registrations?

Hi All,

For the last few days, I have had an increased number of spam registrations at my website domcop.com. Registrations are being made with fake email addresses. I use Google Apps SMTP to send email from the server and all these emails have bounced back with the error "rejected by the recipient domain". They all come from unique ip addresses and are being submitted manually. I can't figure out why someone would do this. They don't get access to the site without confirmation of the email addresses.

Anyone else out there have this issue. Is there anything I can do to stop them? I can't quite figure out how this benefits anyone - even my competition. Am I missing something?

Thank you for your time