I've configured my MT Blacklist to reject any comment that contains a .biz domain in it, since it seemed to me that the only comments I ever saw with that in it were spam. If this causes you any hardship, please drop me a line. I'm thisclose to also banning anything that contains "info@", since a lot of spams also contain that string as part of an email address, but I feel there's a slightly larger chance that a real commenter may want to include such an address (I found one example in my most recent full export), so I'm not there yet.