To be fair, this seems like a generally true case of how subreddits work. Leftist subs brigade, Conservative subs brigade, star wars subs brigade, subs about watching other subs brigade still brigade. It's unsolvable so long as people that disagree are allowed to freely access each other's reddits (not that that's a bad thing).

This is what happens all across the web, regardless of software. Raiding. IRC raids, forum raids, etc. Sometimes it's for fun, sometimes it's more malicious. Most software has ways to mitigate it. Temporarily closing new registrations, invite only registrations, throttling, IP bans, etc. Just like DDoS attacks, there is no true solution to this since that's how the public web and internet works.