I'm concerned that some malicious users of the Stack Exchange network might get 20 (or 30, or 100) people to up-vote every question that they have ever posted on one of the Stack Exchange websites, thus inflating their reputation significantly. Can anything be done to counteract this type of vulnerability?
Moderators have an arsenal of tools at their disposal to detect abusive voting patterns, be it voting rings, revenge down voting, or anything else. And when such behaviours are confirmed, the votes are invalidated and the offending users suspended.
It happens, it's a problem, but we catch them every time, no need to worry about it.