The Less Wrong content deletion policy contains this clause:

Harrassment of individual users.

If we determine that you're e.g. following a particular user around and leaving insulting comments to them, we reserve the right to delete those comments. (This has happened extremely rarely.)

I don't think this really helps you. As you acknowledge, this clause does not actually imply that downvoting is harassment at all. Nor does it imply that blocking users is the appropriate response!

Indeed, the fact that explicitly mentions some crimes (leaving rude comments) and punishments (deleting comments) is probably evidence against this moderation action. If the policy had been totally non-specific, it would imply a wide degree of moderator discretion. The more specific it is, the stronger the implication that things left unmentioned are not actually verbotten.

Furthermore, consider that in the case mentioned in the policy (harassing comments) deleting them is a coherent response which addresses the underlying issue, without very much collateral damage. In this case, banning Eugine from posting does not actually prevent him from downvoting, so the objective is not achieved, but considerable collateral damage is inflicted, by ending his often interesting comments.