For one, if someone takes ownership or control of the commons, they are not the commons any more.
Indeed, having a "commons" is the problem in the first place: no commons, no tragedy of the commons.
Bitcoin has no commons. Every node is owned by someone and they control it, therefore no commons, therefore no tragedy.
For two, it is very much the case that the 'tragedy' occurs absent a mechanism to prevent it. 'forcible ban' or otherwise. You seem utterly backward on the whole 'tragedy of the commons' principle. To me.
That's good, because most economic reasoning seems exactly backwards from common sense.
If land is not owned by anybody, although legal formalism may call it public property, it is utilized without any regard to the disadvantages resulting. Those who are in a position to appropriate to themselves the returns lumber and game of the forests, fish of the water areas, and mineral deposits of the subsoil do not bother about the later effects of their mode of exploitation. For them the erosion of the soil, the depletion of the exhaustible resources and other impairments of the future utilization are external costs not entering into their calculation of input and output. They cut down the trees without any regard for fresh shoots or reforestation. In hunting and fishing they do not shrink from methods preventing the repopulation of the hunting and fishing grounds.
This can only happen when there is a commons ("public property"), which can only happen when there is a ban on ownership. There is no ban on ownership of Bitcoin nodes; each node operator owns his or her node and has control over it. No commons for there to be a tragedy in.
For three, a lot of us, and especially myself, are pretty leery about individuals 'taking ownership or control' over the certain aspects of Bitcoin's function. Indeed, the difficulty of doing this is a major 'selling point' of the Bitcoin solution. In part because of the ill effects of such control in other solutions.
Only the nodes are owned, not "Bitcoin" (the system).