My solution was to remove the registration page link from my skin and form fields within the registration page. I only have the the option to create an account on the checkout, therefore users and spammers will need to place an order to create an account, genuine users will do so but the spammers wont proceed. Has worked well for me for 2 years, no fake accounts created.
I believe they create fake accounts so your shop sends registration emails or password resets to their targets & used to spam people and will give your store/email a bad reputation, some users may report these emails as spam and accounts like google mail will auto junk any future mail you send or your report your site to your hosting provider for sending spam.