Background: During the 19th century the great powers of Europe colonized, and informally dominated, large portions areas of Asia, Africa, and (to a lesser extent) Central and South America. Often, as expressed in phrases like "The Scramble for Africa", colonization was a competitive process, with different nations pursuing their own interests in the same region of the world.
During the 18th century and earlier, this competition sometimes involved war between European nations, both at home and abroad. But so far as I'm aware, this tendency to open warfare declined after the Napoleonic Wars, with the exception of the Crimean War, which I understand was caused in part by fears of Russian colonial expansion. Even then, the western European nations that were involved all fought on the same side.
But the competition for the wealth of the third world must have continued, even if some, like Bismarck, thought that the expenses far outweighed the gains.
The Question: During the 19th century, how did the European nations arbitrate the process of colonization among themselves? How did they censure nations that violated agreements, or skirted the line? What was their course of action if one colonial power pursued a course that threatened to destabilize a region? Or, as with the Belgian Congo, exact an unacceptable human toll?
An Extra Note: I'm mostly interested in the core of Western Europe (France, Germany, Britain, the Dutch, etc.), but the question definitely isn't confined to that. Thanks for your time.