You missed the great Spam Wars of Late 2015. mk and co. fought valiantly and through bravery, determimation, and a bit of hard work they were able to overcome the invading hoards. That said, normal spam is a bit of a thing. Filtering the #spam tag does make a difference as does filtering out users less than 48 hrs. old (an option on your settings page). Both tools are very helpful.
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.
If you come across any of them in the global feeds, feel free to shout out to me so I can take care of it. We think we have a good handle on them moving forward, but about 20 days ago a bunch of accts were made. We think we got most of them, but some snuck through the cracks. Again, we have new controls in place but let me know if some are still around. Cheers!