As noticed, once Reddit builds a search engine, people will flood it with spam. But Reddit is in the unique position that they can figure out who is a genuine user. They have the ip addresses of all the submitters and they have the source redditor of each downvote.
They can still show the comments from VPNs and such. But for their search engine, they can exclusively rely on using the signals from verified IPs.
Instead of building it themselves, they could cooperate with Microsoft and Facebook for the search technology and to know each user even further. Those signals, combined with the downvotes to identify spammers, could create the foundation for Google's nightmares.