make sure that captcha is enabled.
go to Design -> Captcha to make sure the disable Captcha option is not selected.
The registration form will Contain a security question and a Captcha code to ensure that the form is being filled by a human and not a script. This doesn't mean that a human user cant create a spam account, there really is not a way to do that. But by using captcha, it should cut down on the spam accounts.