The current help pages do not make it clear that targeted voting (either up or down) is not allowed.
The only mention of this is in Why do I have a reputation change on my reputation page that says "voting corrected"? which users may only read after their votes have been reversed.
On the other hand users are told when they are awarded the vote up privelege.
Can we please add a section to this page advising users not to target votes at another user's posts?
When should I not vote up?
When a user continually votes (up or down) on many of another user's posts within a short period of time, the system considers these votes to be invalid and removes them.
This could happen for a variety of reasons, such as a user finding a user's great answer and visiting all of their posts to up-vote them, or a user getting into an argument with another user and down voting their posts indiscriminately in revenge.
No matter the cause, this sort of systematic targeted voting is not considered normal behavior and the system will not allow it. The system will automatically reverse such votes and you will see change on your reputation page that says "voting corrected"
If such a voting pattern continues to happen between two users mutually or from one user towards another, or otherwise falls outside of normal voting patterns, moderators and/or developers may investigate the matter; intentionally voting merely to reduce or inflate another user's reputation is considered abuse.
The above could also be a completely new help page "Targeted Voting" in the "Reputation & Moderation" section of help.
See also Please add some basic guidance on voting to the tour for additional justification on why this is needed. The suggestion there is to mention targeted voting in the Tour.