There are two ways to stop a Pokemon from evolving. The first is to give it an everstone, which prevents you from working everytime the Pokemon is ready to evolve. The second way is to press "B" when the Pokemon is evolving. If you did neither, then your Pokemon is not ready to evolve.
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The everstone stops evolving all Pokemon that evolve in the first place.
Give them a everstone or press b when they are evolving ;)
Press b. Or give it a everstone.
Nothing. An everstone is just a hold item to stop the Pokemon from evolving.
If you stop a Pokemon from evolving, it`ll try to evolve every level after that unless it`s holding an everstone.