So scratching the previous arguments, his next argument was that instead of shooting up the Target, what would prevent Wal-Mart from putting up tons of billboards and advertisements saying "if you go to Target, you'll be killed!" or "200,000 people died just stepping inside a Target store!"
The same thing that stops them from hiring mercenaries to kill Target employees. As a consumer, if I know that my best shot at survival and happiness is not to give my money to companies that exercise force or fraud, then I will act accordingly. Since those companies need money to commit force or fraud, they will quickly go out of business.
Now, it's possible that they will try to commit force or fraud in a clandestine fashion. As a consumer, I am concerned about this, so I would also be sure to only give my money to companies whose financial records are audited by reputable third-party auditors, or whose activities are investigated by third-party detectives, etc.
For every new thing that your brother can dream up that would make the free market not function, I can create a business whose product is to solve that problem in the free market. If it is a legitimate threat, then I should be able to successfully market my product to consumers and get their money.
Keep in mind that it is not the anarchist's responsibility to come up with a solution for every imaginable problem in the world. There are some problems that will not necessarily be "solved" by the free market, especially if they are not currently solved. When debating with people who keep throwing out problems with voluntarism, when appropriate ask them "well, is that problem currently addressed?"
Our roads are horribly inefficient, starvation is rampant throughout the world, and public school students know next to nothing other than state propaganda, so these kinds of objections are irrelevant if voluntarism can achieve only par for the course. (And I would argue that, typically, it can do far better.)
Also don't forget the power of arguments from morality like the "Against Me" argument.