I can see several strategies you could use under conditions you described to maintain your monopoly if you're already big. You could, for instance, immediately buy out any serious competition before it becomes dangerous. Or, since you're a big player with deep coffers, upon seeing an upstart you could start operating at a loss an just wait until your competitor runs out of capital, and then bring the prices back up. Defeating you would require a lot of people coordinating to hit you at the same time, and we all know that people suck at coordination (and if somehow they managed to orchestrate such an operation, it wouldn't take much to bribe a participant and turn him into a defector in order to derail the whole group).
Or you could just hire a hitman. There's no concept of "fair play" in the "physical order of nature".
Or you could just hire a hitman. There's no concept of "fair play" in the "physical order of nature".