The fable in the OP forgets the fact that since the barge-men were doing well at the time the wall was built, they could have just negotiated with all the land owners in between their two towns and built a canal and the contracts would have priced in reasonable tolls anyway.
Besides this, it is likely that the barge men would also have had the option to buy up all of the actual river property on the way down the river as a consortium and the wall builder would have been given the choice of sell it or make a contract of rental, which would have made him happy and money anyway.
This story is typical of people who think in terms of the commons still existing in an anarchocapitalist society. This cannot happen. It interferes with business, and despite what these anti-capitalists might think, it is the whole purpose of traffic routes to allow traffic through because it fosters trade between people along the path. In as far as money is needed to maintain the route, there has to be some scheme of gathering money from those who benefit from this route. Those who use it to do business with those people pay it anyway through their trade with these stakeholders.
So you favor the abolition of the commons? Fascinating. Everybody else who replied to my post said "well, at least you have to keep the commons." I was about to write a post discussing this. In fact, hang on a sec - I just did. For a discussion on the commons, check out my new discussion on the commons.
Of course, your answer is the right one. Your solution to the coordination problem is to coordinate. To organize, to unionize, to band together to exert the functions of government (though, under a different name). And then pray that it all stays non-violent.