[mkempe]

Make it a high karma cost. E.g. you must have been upvoted 10 times to be able to downvote once. That cost can climb up for people who generally don't significantly upvote more than they downvote.

[added] I've noticed that sometimes in a deep conversation between only two commenters -- where they cannot downvote each other -- one of them gets their comments downvoted within a minute of posting or so, suggesting that there are parallel accounts used to downvote as a method of intimidation or retaliation.

[yasp]

Saw you getting wrecked by ideological downvotes in a thread I made recently. Good job not letting it keep you from speaking.

[AnimalMuppet]

I've seen something that may be related - a comment abruptly getting downvoted twice. That can happen just by coincidence, but it seems to me that it happens too often to multiple comments in the same thread.

Maybe we need something like keeping track of the IP address of downvotes, and seeing if multiple downvotes to the same comment come from the same IP address? Or seeing if two accounts have a strong correlation of downvotes?