Comment by Hedja

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.