If the offender really is at fault (which should be quite easy to tell in most cases), then they should probably be banned since this is a pretty disruptive behaviour.
At any rate, have you checked with Eliezer - he used to claim that it is impossible to check a user's voting history, so he might have some other plans that you are not aware of.
I'm figuring that he'll see this post sooner or later.
Below is a message I just got from jackk. Some specifics have been redacted 1) so that we can discuss general policy rather than the details of this specific case 2) because presumption of innocence, just in case there happens to be an innocuous explanation to this.
So... thoughts? I have mod powers, but when I was granted them I was basically just told to use them to fight spam; there was never any discussion of any other policy, and I don't feel like I have the authority to decide on the suitable course of action without consulting the rest of the community.