I want to write a story about a place called Kingdom Come, where you can come in but you cannot go (i.e. you are not allowed to leave). Has this idea already been taken? It really sounds like the sort of idea that has already occured to some writer. If so, can I considered a plagarist?
The answer to "Has this story idea already been done?" is always yes, regardless of what the idea actually is. Everything's already been done. In your particular case, the one example I can think of immediately is the TV show Once Upon a Time, where anyone who enters the town of Storybrook becomes unable to leave due to mysterious forces (there are probably better examples, that's just the one that came to mind).
Fortunately for you, ideas are not protected by copyright and cannot be "taken"; one person cannot claim a story idea and then prevent anyone else from ever using that same idea. So it doesn't matter one iota whether your idea has already been done or not. As long as your story isn't a scene-for-scene copy of an existing story, nobody could possibly accuse you of plagiarism.