right... 2016 wants there meme back. for everyone else reading this if you Digimon had a black circle with a cross through on their portrait 🚫 this means they're blocked from evolution.
There are items in the shop that can be unlocked via the medal challenges I forgot which ones exactly but they also give you one of those anti evolution items for free. DO NOT GIVE IT to your Digimon unless you don't want them to evolve but also to remove it is 150,000