edit: I just looked at your profile and the deletion/banning of a user account will remove rep too, so I suppose in that manner a mod can "reverse" voting, but it's all or nothing. They have no way to flip individual votes.
edit2: Moderators are not employees of Stack Exchange, they are volunteers elected by the community. Of course employees can change whatever they feel like. This discussion is about the moderators.
AFAIK, under no circumstances can a Stack Exchange moderator reverse a vote.
More info on the serial vote detection process here: http://meta.stackexchange.com/questions/126829/what-is-seria...
edit: I just looked at your profile and the deletion/banning of a user account will remove rep too, so I suppose in that manner a mod can "reverse" voting, but it's all or nothing. They have no way to flip individual votes.
edit2: Moderators are not employees of Stack Exchange, they are volunteers elected by the community. Of course employees can change whatever they feel like. This discussion is about the moderators.