Right now, the only tool we have against abusive downvoting, is the higher weight given to high rep voters. We are observing this is not sufficient against abusive downvoting circles, who seem to carry enough weight to cause real, malicious damage. If you require explicit examples, I can provide them privately.
The influence of rep on voting weights is not linear at all, but seems to be quite exponential. While this could be pushed further and further, we also could run into the problem of high rep individuals becoming too powerful.
Not only is that bad for our democratic aspirations, but It is quite apparent that at least one high rep individual is engaged in abusive downvoting. Further empowering abusive high rep votes can invalidate all our efforts, since one abusive high rep vote becomes worth so many low rep honest ones.
I think the only real solutions involve the Narrative Company announcing that Narrators will soon be able to lodge complaints for abusive downvoting, and such complaints will allow the Tribunal to view the votes for a post, and gain access to relevant voting data of accounts involved in the downvoting.
There should be a statute of limitations on this, of something like 6 months - in any event, long enough so that the functionality arrives in time for people to lodge complaints about violations that occurred this month. The knowledge that abuse carried out now will have consequences in the near future will have an immediate and beneficially cooling effect.
To reduce frivolous complaints and to fund the work this creates, there should be a NRVE fee involved in lodging a complaint. If the Tribunal finds there has been abusive downvoting, the abusers could then be made to cover the fee, and to pay penalties on top of the fee, in the form of docked earnings. Significant reputation penalties should be possible as well.
The complaint process should involve an opportunity for the downvoters to explain their downvotes - these explanations would aid in a determination.
Within the course of a complaint, it should be part of the scope to determine whether there might be voting circle downvoting involved. Once abuse is found to have been committed by several accounts on one post, if it is found that these same accounts have acted in concert, abusively, elsewhere, a new level of more severe penalties should be considered.
These measures will be effective because abusive downvoting only works if the voters have enough rep to do damage. The mere threat of removing the rep that gives them their power, will stop most of the activity, and will effectively neutralise the power of those who persevere. Add to this the penalties that limit rewards for infractions, and you have a serious incentive to vote honestly.