Thanks for asking. There are lots of different schools of thought on the matter, so I'm just giving my own thoughts on it here, not an overview of every possibility entertained. If you were to strictly-speaking enforce a process of diminishing returns on concentrating wealth, that wouldn't be a free market anymore; keeping wealth from concentrating in a free market requires you find some kind of loophole or process that is enabling that, and then you just stop enabling it.
I think that that loophole is rent, including rent on money otherwise known as interest. I'm cutting this really quick here and glossing over a lot (mostly rebuttals to anticipated objections), but basically rents happen when one person has more capital than they (in their own estimation) need for their own use, and someone else has less capital than they (in their own estimation) need for their own use, and the one with more exploits that difference by charging the one with less a permanent fee for the temporary use of the capital, so that when the whole transaction is over, the one with more now has even more, and the one with less now has even less, and (this is the key part) that process then loops over and over and over again, unless by some other means the one with not enough capital manages to get enough capital of their own (a process made all the harder by having to pay to borrow someone else's meanwhile).
In the absence of the ability to rent the capital (lend it at interest), the one with more capital would need, if they wanted to profit from it at all (since they're not using it themselves), to sell their excess capital, and the only buyers would be those without enough already, so the sale would have to be on terms (price and payment schedule) the latter can afford, to satisfy both of their own self-interests. This would still come at the cost of extra labor from the buying party (being all that they have to trade, lacking in capital), and consequently afford the selling party some leisure (lack of labor needed), but that would be a temporary condition until the sale is complete. Afterward, both parties have the amount of capital they (in their own estimation) need for their own use, and can continue about their usual amount of labor, keeping the whole product thereof for themselves.
For illustration consider a toy model economy where the only capital is land, the only labor is farming, and there are only two identical participants, with the same needs and labor capacity. If they both have enough land to farm to meet their own needs, then they can each farm their own land and they are equals. If one has more land than he needs and the other less, and rent is a thing, the first can rent some of his land to the other, which he pays by also farming some of the other guy's land (besides the part he's renting), meaning the other guy can kick back and work less... and this will continue like that indefinitely, one person perpetually indebted to the other, all because one of them started out with more than the other. Meanwhile without rent, the poorer farmer would still need to buy the excess land from the richer farmer, meaning he would still have to do some of the farming for the other farmer in trade for that land, but a finite amount thereof, after which time the price will have been paid, they will have equal amounts of land, and go about farming it with equal amounts of labor to meet their equal needs, neither indebted to the other.
Fortunately, we don't have to ban rent to get rid of it, because rent is not just a voluntary trade of one thing for another, that would be a sale; it's a special kind of contract. If we simply stop enforcing such contracts, consider them invalid the way we consider a contract selling yourself into slavery invalid, then rentals and interest-bearing loans would be, while strictly speaking allowed, economically untenable (inasmuch as I will not be punished for agreeing to be your slave, but as soon as I decide I'm tired of it, tough luck for you, that agreement is not binding). So you're not using any force, and all trades are still free, just certain kinds of contracts are not protected, and so won't be used, and in their absence, actual trades (still free) will have to be used instead, in the process circumventing the problems that arise from rent.
And Marx thought capitalism was an unavoidable byproduct of free markets just because he couldn't see what it was, in addition to the free trade of private property, was really causing it. The problem is not, as Marx thought, that the factory-owners pay the workers less than they in turn sell the product of those workers' labor for; the problem is what causes the workers to be in such a bind that their best option is to agree to such an arrangement instead of telling the factory-owners to go fuck themselves, and that is that the workers have debts to their landlords (and possibly other creditors) to pay. The only difference between capitalism and feudalism is that the lord whose land you live on and the lord whose land you labor on can be different people (and you can pick who they are); you're still laboring upon someone else's capital, and then giving a big chunk of the product of that labor to someone to borrow the capital you need to live and continue laboring. That the someones are different instead of the same doesn't make that much difference to the economics of it.