I think these are fake accounts using delegated Steem Power that they received from new account creation. It seems like the delegated power should be removed in this case or perhaps we should not let downvotes be cast by accounts who do not have any Steem Power earned of their own. Another possibility is to only allow downvotes to be cast by accounts who actually have a post waiting to be rewarded during the seven day reward period.

There seems like there are some solutions to this but there does not seem to be much action being taken over it. In my opinion it is not really good for a social network to permit such behavior because it tends to be annoying and does not encourage cooperation.