Your favorite Pokemon evolving is one of the coolest and best moments in the long running series. At least, that’s the case most of the time. Sometimes you like your Pokemon just the way they are, thank you very much. And when this is the case, you want to stop it from evolving at all costs. Thankfully there’s an easy way to do this. Here’s how to stop evolution in Pokemon Let’s Go Pikachu and Eevee.
There’s two parts to this question. First up, if you have a Pokemon who is ready to evolve and you want to prevent this from happening right now you just have to press B whenever the occasion comes up. It really is that simple, with the player just having to hit B, or the big red button on the Poke Ball Plus, to stop the whole process right then and there. Of course, then you might wonder how permanent this fix is.
The answer is just until the next time the Pokemon levels up. Any level gain will prompt another evolution, which you can again stop with B. You’ll unfortunately have to keep doing this throughout your time in Pokemon Let’s Go Pikachu and Eevee, because the second part of this question has a much less easy answer.
There is no way to stop evolution permanently in Pokemon Let’s Go. Where previous games had the Everstone, an object that Pokemon can hold to prevent evolution forever, Let’s Go has nothing to replace it. So, if you want to stop your Pokemon from evolving, you have to either remove them from your party, or be vigilant with those presses of the B button.
Source : https://attackofthefanboy.com/guides/pokemon-lets-go-how-to-stop-evolution/