Security problem with WP 2.1

Hi,
I’ve set in the settings that anyone can post comments once he/he is registered user. However, every day I receive spam in my blog’s comment fields from the same IP address 209.160.64.236 and each comment has differnet user name and different content adn differnet link, but these user names are never registered in my blog.

Author : online-streaming radio stationsrock stations
Author : candy bouquet nebraska
Author : tavola+pace
Author : biaxin xl no prescription
etc.

So there must be some way spammers can post spam without really registering with my blog. Two suggestions:
1) add feature to block specific IP addresses;
2) improve registration autentication, so that spammers could not spam without taking real registration.