Spam users infesting my site, bypassing the registration form!

Hi there,
I haven't even started advertising my site yet but I have had 10 new users registering since yesterday (with nonsensical usernames and weird numerical email addresses but no other info). They are clearly bypassing my registration form with its required fields and sign up code....Luckily they haven't created any sites which I assume is thanks to Anti-Splog. But I am still worried that the site is not secure and that they may cause some damage...Can you help?
Betty