There are a few steps to removing a Pokémon from Isle Evelup.
Note: Some other islands, like Isle Abeens, naturally has Pokémon roam around that are in you PC Box, and cannot be taken out. However, this doesn't apply to Isle Evelup
Tap the sign on the island
After Clicking the sign tap the button that allows you to take back your Pokémon. It should say something in the realm of "Take Back."
This will lead to your PC Box. Drag the Pokémon that you want to take out from the party space into your boxes.
Confirm your choice and exit!
This should be how your remove Pokémon from Isle Evelup!