I think they can, actually. I had a disagreement with someone on SO; this person subsequently went on a downvoting spree on my previous answers. I wasn't terribly worried about the points but e-mailed the mods to report the behaviour. I was told they would reverse the downvotes if the user persisted, but that they preferred not to get involved (for obvious reasons).
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.
Moderators can ask someone from the SE community team to look into it, and they can see individual votes and invalidate votes between certain users. The only cases where this actually happens are users using sock puppets to upvote themselves, colleagues/friends/family going through every post and upvoting everything or users targeting a specific user with a large number of downvotes.
The one thing they all have in common is a larger pattern of votes. SE does not act on individual votes, and emailing them about those is a waste of time.
The term "mod" is a red herring here. The engine will auto-reverse obviously targeted voting, and the community team reverses cases too subtle for the engine to handle, but that are clearly personally targeted.