This house was never on the MLS. It appeared as an open listing briefly several years ago, and was still showing up as for sale.
In this case, the agents are easily identified by finding the house on the websites of the firms that had it as an open listing. It is then a simple matter to contact those agents to request that the posting be removed from their systems.
In any case, we have begun to take care of it. It is being taken off our site and off the sites of several other local firms. That is the first step to making sure that no aggregators can pick it up.