One thing that reddit gets "for free" is a user authentication system that is tied to their existing system. A huge way of them slowing down spam was preventing

One thing that reddit gets "for free" is a user authentication system that is tied to their existing system. A huge way of them slowing down spam was preventing anyone from editing unless they had a reddit account that predated the existence of /r/place. You could mock something similar up by OAuthing to Twitter/Facebook and only allowing accounts created prior to today from editing.