If they set their username to their name that's not really any stopping that without code based validation to compare username against name, which is doable with CB Code Field since it provides Code Validation. Sounds like you're just getting spam registrations though which you can help protect against by using captcha provided by CB AntiSpam. Aside from that I'm not sure what kind of validation you're wanting.

Do you not have the username field available for them to actually supply a username? If not it's using the fallback behavior, which you can change what it falls back to within CB > Configuration > User Profile. Otherwise it seams odd they'd supply firstname and lastname also to the username field unless that's what they really wanted. Regardless no it's not possible to do that without some validation that can execute custom code so you'd need CB Code Field to do that.