Few options: make system more complex, add randomization, increase moderation effort or make it less open.

I am fan of adding randomization. It was previously proposed: show few random new items on a front page in random places, maybe with a little threshold.

I was thinking about community page destined to fork. After community reaches certain threshold (size, activity) there is automatic fork and 2nd generation community is created. Some algorithm adds accounts for certain users and blocks some other users from account creation. It's just a pipe dream...