Unlike all other spells evolving wilds will not give you the option to hold on to it.
Many cards in the game don’t ask if you want to cast them once they’re filled. Since the spell doesn’t deal damage or affect creatures, I’m not surprised Evolving Wilds is one of these. If, before you make a match, you touch the Spell symbol in your hand so that it dims, you can disable casting it just like any other card.
Wow I never knew I could turn off cards, thanks!