Spam registering

I use the sidebar login and so far no problems at all. But from about a week ago I get this spam user. He/it keeps registering himself over and over with same username and email: Boriliniofe and goolvezedvect@yandex.ru. I keep deleting this user but 10 minutes later it registers again. A real pain in the butt. How can I block this emailadress/user?? Is there any other way preventing spam users registering themselves??

http://wordpress.org/extend/plugins/sidebar-login/