They can use the existing structure of peering and transit agreements to control to whom they are willing to provide service, and possibly to charge for the service. In practice, however, such routes are almost guaranteed not to work. The actual limit may vary from network to network, and also over time. It could indeed be argued that using a large network is a waste of the precious addressing resource. However, this is a waste for the good cause of actually moving to IPv6, i.e. providing a real relief to the address exhaustion problem.
Erroneous advertisements are a frequent source of errors in inter- domain routing.
A misconfigured AS will advertise that it can reach some random network, divert the traffic, and effectively cut that network from some parts of the Internet. As a protection, many managers of border routers use databases to check the relation between the advertised network and the last hop in the AS path. Some have expressed a concern that, while the assignment of an anycast address to 6to4 access routers would make life a bit easier, it would also tend to leave things in a transition state in perpetuity. In fact, we believe that the opposite is true. So the anycast proposal actually ensures that we don't stay in a perpetual transition.