This is a question that can be asked in any Meta but I decided to ask here as it happened here. I won't name anyone but you can see I have only one answer so you will know whom I am talking about. I answer a question and get two upvotes. Another guy answers the same question and cites my answer and got two upvotes-one from me. Now this third guy comes up and he just gives an example, not giving a good answer or anything so I don't upvote him. He instantly downvotes our answers so it is now 1-1-0 (0 is his answer). I don't whether someone upvotes him or whether he uses an alternate account to obtain an upvote...but now he attains 1-1-1. So from 2-2-0 to 1-1-1. He now has better chances to get his answer chosen and upvoted. Is this fair? Should this be reported?
Just let it go. Upvotes and downvotes happen, and the only thing you can do about it is post quality content to encourage people to upvote.
You shouldn't flag it. People are free to vote how they want, and besides, you don't know who actually cast any of those votes. As a mod, I don't even have access to that information.
This has been discussed at length over on the main meta site:
The consensus has always been not to try to block it.
Note that people are encouraged to upvote rival answers on questions that they have answered. Do it 100 times and earn the Sportsmanship badge.