Quick fix: temporarily require a Gmail address for creating a new user—Google is good at preventing massive account creation.
Allowing new users to post only a single comment, which will have to be approved by a mod to make it visible and to unlock the ability to comment freely.
Maybe, put every new user who receives an downvote but no upvotes from users with >100 karma into a mode where their comments are only visible by mods and themselves till the mod approves the user?
[Brainstorming]
More general solutions:
Quick fix: temporarily require a Gmail address for creating a new user—Google is good at preventing massive account creation.
Allowing new users to post only a single comment, which will have to be approved by a mod to make it visible and to unlock the ability to comment freely.
Maybe, put every new user who receives an downvote but no upvotes from users with >100 karma into a mode where their comments are only visible by mods and themselves till the mod approves the user?