When a world is generated, you are given the option to disable structures under the world options menu. Nether Fortresses are structures, but they are not disabled. I have not included any images, as there is no reason to post random images of Nether Fortresses. This is annoying for custom maps, where Blazes/Wither Skeletons are spawning where they are not supposed to.
Duplicate of Unresolved
MC-16387
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