On Stack Overflow, I have tried to consistently vote for some of my friends' answers, since he generally provides good answers to somewhat obscure questions but doesn't get a lot of credit, since they're not terribly popular. One day I noticed that his reputation dropped a ton; upon further inspection, I noticed that all the votes I'd given him in the past had disappeared.
Since I am a long-time reader of Coding Horror and I know Jeff has weird ideas like "you can sanitize HTML with regular expressions", I just assumed that Stack Overflow had randomly lost a ton of data because it's full of bugs ;-). So I dutifully upvoted everything again, burning several days' worth of votes on this rather than voting for other worthy things, hoping to work around a bug or data migration issue.
Personally, I don't regard this as "fraud" at all. I'm voting for my colleague's answers because I think they're good answers; I discover them through his list of answers, but I vote for them because they're good. I'm also not the only one upvoting these answers, either.
I don't think I do anything that is fraudlent. I don't vote exclusively for my friends. I legitimately participate in the site: I ask questions and I answer questions; I upvote good stuff and I downvote bad stuff.
So, while I respectfully submit that the administrators of this site and I could agree to disagree that this is legitimate voting behavior, as a legitimate contributor in other respects, I would have at least appreciated an error message telling me that my votes were being denied because I'd voted for this person too many times. Maybe a nice hint like "You've voted for so-and-so too many times. Give somebody else a chance!", with a link to the main questions page. If those are the rules, then okay, I'll play by the rules; but this is more like getting a foul in a competitive sport, and instead of a referee calling out a red card or whatever, the scoreboard just changes silently with no word from the announcer, and sometimes the other team leaves without a word.
(PS: please don't quote joel's "Not Just Usability" at me as an answer here. I am not saying that the site should disclose all information to all users at all times; I can understand that sometimes information should be hidden from attackers. But a real abuser here would have much more quickly resorted to a fleet of sock-puppets spamming from different IPs, rather than assuming that the system's algorithms were otherwise reasonable and would eventually recognize that my interest / votes were genuine.)