as far as I know, If a user is Blocked and Muted by enough people, they don't show up on the main hubski page. You'll still see their posts if you follow them, but the community has made it clear that they aren't the face of the site. as for brand new accounts, I'm not sure. the blank front page might be set up to not show users less than X hours old.
The filter new users only filters a new user for the first few days they're on this site. If they're legit, they'll keep on posting. Since most spammers seem to only post once or twice and then fade into the nethers, the filter new user tool works great for them. People who don't have usernames see everything that shows up on global. New users. Spam posts. Everything. You have to have an account that's filtered. As coffeesp00ns pointed out though, if enough people filter or block a certain user, no one, not even unregistered users, will see their posts unless they follow them specifically.