Let's say hypothetically that Hubski grows by a large factor. Wouldn't this system become a huge burden to users as trolls and children start spamming links and memes with no way for other users to "downvote" junk material?
1) I can ignore new posters for 48 hours. So trolls and children best have one hell of an attention span as it'll take two full days of childishness for me to care. You'll note even Reddit's enfants terrible are losing steam. 2) I won't see your links and memes unless they're tagged as something I follow, or unless I browse global. 3) We had that. At one point Lorelei and a bunch of the SRS crew came by. Unfortunately they found they were mostly talking to themselves because of (2) so they eventually left. And that was before we had (1). Mostly, our problems are traffic. Bad actors really don't have the impact here that they do elsewhere because of the site's dynamics.
To add to what Quatrarius has said, if there is a user who has a threshold ratio of ignores to follows, then their content will not show up in casual browsing (via "global"), so it is effectively invisible to the majority of people, short of someone else who isn't globally filtered sharing it. This is the way in which the community can help to keep the place clear of spamming.
That's where muting, filtering, and ignoring come into play. You don't need to see people's posts or allow them to post on your posts.