They are not registering in your *theme*. You are using Buddypress with open registration.
Go ask in the Buddypress forums. If you were using Genesis Connect, we have a spam stopper in that.
These forums were CLOSED to new posts on November 19th, 2012. New forums are here: http://www.studiopress.com/forums/
and you will need to SIGN UP AGAIN.
If you need assitance with setting up your theme, go to http://my.studiopress.com/
and read the SETUP links or the TUTORIALS link on the side tab.
If you need Account Assistance
, file a help desk ticket at the same location.