It seems to me that this problem has had a solution which has stood the test of time for millennia. Shunning. Whether it was ancient Babylon or 19th century London the act of shunning someone (for specified periods of time, even life) has achieved the goal of weeding out undesirable individuals from the group.
All that is needed in the web world is a point system for members (which you already have). Members which contribute the most have the most points. These people’s opinions count the most. If a high point user shuns another user, that shunning counts severely against the shunned. Also if a high point user is shunned the shunning counts less against them. High contributing members may be attacked or maligned at random and sometimes this means the user is actually doing a good job. In other words, the users held in high regard are protected more from random shunnings.
When a user hits a predefined threshold of shun points vs. Their esteem points they are shunned by the entire group for a preset period of time and told so. The shunned are always overtly ignored. With each subsequent shunning more time is added until a shun threshold is reached at which point they are excommunicated.
The shunned and excommunicated always have at ther disposal the petition system. They may ask for forgiveness (or point out the error of the shunning like “someone stole my account.”). The petitions are sent to those that did the shunning and they may decide to not shun them anymore.
This process works orit would not still be in use by churches and organizations around the world.
Just a thought.