First, monopolies like Microsoft and Apple rely very little on patents. They don't even rely on copyright, because it doesn't provide them sufficient rights, in their opinion. They rely on contracts. Second, they'll mostly likely fail no matter what. Unless they can keep innovating and providing the products their customers most want, they'll be eclipsed by a more agile competitor.

The most effective way to maintain a monopoly in a free market is to be the most efficient company, providing the products your customers most want at prices they are happy to pay. Some market segments are 'natural monopolies' in the sense that they have unusual factors that make monopolies more likely in those segments. A good example is operating systems -- there is a benefit to running the same operating system many other people are running even if it's slightly inferior for your particular application. In these areas, a monopoly can be a little bit worse than their closest competitor and still maintain near-monopoly market share.

But if we all benefit from running the same operating system, even if it's not the very best one, why shouldn't we? Until a competitor is sufficiently superior that it's worth the cost of switching, of course.

There are market failures that can occur in a free market where you can have near-monopolies even when it's not particularly efficient. But they are so rare as to be almost non-existent compared to regulatory failures. (And, of course, advocates of free markets believe that it is entirely appropriate to use force to break up monopolies maintained by force or fraud, just as it is appropriate to do so for non-monopolies maintained by those means.)

It's hard to take this kind of comment seriously, but just in case, the answer to your question is: The prohibition on the use of force except in defense and the requirement that people honor their agreements.