The problem of rent-seeking is by no means an exclusive problem of a propertarian anarchist order. On the contrary, it is the defining problem of a statist one. The state is essentially a gigantic, parasitical, rent-seeking entity, with a dash of extreme violence mixed in.

You say that a voluntaryist order neither offers nor proposes solutions to this problem, and yet one such solution has already arisen in the form of easements. These are a limited grant of access to someone else's property, especially for the purposes of unimpeded travel through it, based on either agreement (contract) or pre-existing usage. In fact, in the situation you describe river travel and trade has gone on long before Ben decides to enforce tolls, so you almost surely have people contest his right to bar the river due to that long-standing usage. Even from a hyper-autistic atomistic libertarian perspective, they could easily claimed to have homesteaded access rights to the river, and not even invoke concepts like easements.

The other problem with your scenario is that everyone on this island are so pig-headedly selfish that they would actually prefer to make themselves poorer and destroy their industry just for the hope of an ever-diminishing profit. As trade declines, they will have fewer and fewer people willing and able to pay the tolls. Who would act like this, especially in the face of competition from other sources (say, an enterprising road builder or a canal trust that diverts water access away from that part of the river)? If people actually behaved like this, a sophisticated market economy would simply not be possible.