There is 2 ways to stop evolving: During the Pokemon's evolution keep pressing the B button to stop the evolution, also have the Pokemon hold the item everstone no matter what level the Pokemon is it will never evolve if it's holding that item and if it evolves during trade if it's holding the everstone it still will not evolve.
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Give them a everstone or press b when they are evolving ;)
you have to trade it to a friend and it will evolve then your friend will give it back to you!!!!!!!!!!:)hoped i helped:)
The everstone stops evolving all Pokemon that evolve in the first place.
If you stop a Pokemon from evolving, it`ll try to evolve every level after that unless it`s holding an everstone.
so far, you cannot evolve eevee either to umbreon or espeon on FireRed because on FireRed and LeafGreen, there is no clock system. You must trade eevee to Emerald for evolving to Espeon or Umbreon