I don't agree. Radio in the early 20th century in the United States was not regulated, and operated in the unregulated fashion you described, and it was a failure. What has really changed in the intervening century? Spectrum interference is still as much of a problem now as it was back then.
Allowing anyone with a small amount of money to disrupt valuable uses of spectrum would basically legalize extortion. I doubt many radio-spectrum businesses could exist in an environment where any random person with a few thousand dollars and an evil mind could legally demand that they pay up or be shut down.
You just called it what it is. Criminal extortion, and it would also be sabotage, to use a frequency band to attack another party running on it.
That doesn't magically become legal. For the same reasons you could take someone using "your" frequency right now to court, you could take someone using the frequencies you do only for the purposes of attacking the signal quality on the same basis.
And no, opening up spectrum doesn't suddenly mean everyone and their mom is buying radios and spamming electromagnetic radiation, for the same reason people don't do it today, because they can be targeted for sabotage if they aren't productively using their radio usage.
I don't understand, if there's no ownership of the spectrum, then on what basis would the user be able to take the attacker to court? What if the attacker says he just really enjoys spamming high-powered electromagnetic hash, but would be willing to give up his hobby for the right price?
That right there would be considered extortion in court, and like I said, it is sabotage to actively attack infrastructure to defraud a business (ie, blocking roads for trucks, building a power sink to rip juice from a local area from someone, scheduling trains to intentionally prevent someone else from using them at a certain time).
It is like any other infrastructure. You should be free to use it, and punished if you abuse it.
It doesn't require the violence of law for people to come to compromise. Profit motive is plenty enough I think.