Steem blocks vote operation for a handful of accounts labeled as "malicious voters". Namely, @acidyo, @infovore, @spaminator, @steemcleaners.

https://github.com/SteemLabs/steem/commit/4116c0ee99f4d13f3583cc4b9967418caa76a58d#diff-9b0390bcfbe99179cc655a9aed09bd2472743ef120c64b6ddaec50a66d9431deR103