Hi! At the moment, the Fusion system is part of CDS’s core mechanics, so there isn’t a direct option to completely disable it.
However, if you don’t want to use Fusions in your game, you can simply avoid creating any Fusion recipes, so they effectively won’t be part of your gameplay.
Keep in mind that the Fusion system is also connected to some interactions such as playing compatible Equip Magic Cards directly from the hand, so completely removing it would affect other parts of the system as well.
I may consider adding more control over this in a future update, but for now you can simply build your project without using Fusion recipes. :)