To prevent this kind of thing you actually need to track reputation. With a reasonably strong online identity mechanism, true reputation can be tracked and bad actors can be kept out of a system. Without reputation, people are effectively anonymous and will behave accordingly.
To put this in another context, imagine you have a grocery store. One of your competitors sends an employee into your store to knock cans off your shelf. You kick him out and tell him not to come back. But he puts on a cap and comes back. Each day he puts on a different hat and tries to come back. In this case, it would be easy to keep him out of your store.
It's not easy online because we don't have strong online identity and without that, you can't have reputation.