I think it could certainly be wise to implement a limit on the rate at which one can downvote posts by a specific user, or, if that's technically difficult to implement, the rate at which one can downvote fullstop.

The more involved measures you suggest would require effort, but I suppose the question becomes: what is LessWrong for? If it's actively for improving rationality, such measures could be worthwhile, assuming we could find or reroute some moderators / mentors / monitors.