Spam is automated. Little robots troll the internet looking for blogs on which to lead their spammy turds.
Turn on Akismit, install cookies for comments, or bad behavior (both are plugins), and delete the spamemrs.
What determines whether a post is spam.. Does WP look to
see whether they are registered to the blog..
If you have Akismet turned on (or some of the other plugins), it checks a database to see if the commenter has been reported as a apammer
As for cookies-for-comments, it works great. It works by checking to make sure your real comment form is being used before allowing a comment.
Many spammers do dot use the real comment form. They are bots which post through code directly to your comment link
Checking to see if a user is registered to your blog does nothing. On VoodooPress I used to have 50 people a day sign up on my site, and nearly all of them were spammers. So spammers will register to your site trying to get their links out to the world
What i do with my online forums, is block proxies for like a year out, and give some nonsensical reason, since I know the spammers are just being idiots..