I have already devised a scheme for this. It's central to the changes, in fact:

https://github.com/calibrae-project/calibrd/wiki#reputation-coefficient

The exact details of the implementation may change a little, and I haven't yet fully studied the way reputation works as it is but the principles are as follows:

• Reputation acts as a limiter on bandwidth. When an account drops to zero reputation, their bandwidth will be so small they can only make the equivalent of 1x 1kb post, or maybe a couple of votes, transfers, mute/follows, account data changes. An account like berniesanders' -16 rep could not exist, because there is no point in reputation dropping below zero anyway, but if it did, the account would effectively be the same as deleted. I will be examining the way the algorithm works, I don't see any reason for a negative at all.
• Reputation scores act as a limiter on stake. Only the accounts with the highest reputation have no limitation, the rest scale to this level, from the entry level account baseline.
• Reputation values are calculated independent, entirely, from stake. It is possible stake currently has some relation to reputation effect. This will be changed. Reputation interacts with reputation, alone.
• Mute and Follow create a temporary effect so long as the mute or follow holds, that acts like a down or up vote on every down or upvote from an account, that is proportional to the reputation of the muter/follower. This effect decays, of course, with the number of followers, as you can see on the wiki, in an inverse square, which will work to slow down how fast a ganging-up of accounts will work. With sufficient mutes on an account like berniesanders, if he was still above zero, he would go to zero, and it would be the same as if he had been flagged down.
• Because there is no self-upvoting, it is not possible to directly elevate reputation score. Of course. If users discover evidence that two accounts or more are controlled by the same person, they can publish the evidence and if the community decides this is a sybil account, they can immediately mute the accounts, as well as downvoting every post made by all of them.
• At zero reputation, the account will also be unable to transfer more than 10% of its funds per day. This is to encourage the account holder to make amends with the community. This will be especially punishing to large accounts like berniesanders' many puppets.
• Unlike steemit, the display name will actually show properly everywhere, so there is really never any reason for a human to have more than one account.

Calibrae will leverage the ability of humans to recognise sybils, scammers, trolls and scammers, and allow the community to judge them by suppressing their ability to operate their account, both through downvotes, as well as being able to effectively downvote the account using mute.