One goal of #Hubzilla
is fighting against spam.
Interesting looking at the global directory #^https://hub.libranet.de/directory
You see lot of account created for SEO purpose. I don't know if it works. It is an easy link for free. You create an account on each hub and you add a link.
I think the community didn't work on that. I was an admin in the past and the only solution I found was to close directory and open it only with invitation. But invitation is very rustic and manual. Not as easy example read automaticaly the list of contact in the phone and invite them to register and follow.
It was just my early reflexion