How do I disable the Cities of Tomorrow Expansion for SimCity?

From what I know, sadly, there is not a way to disable any content in the game. Since what you have is the Complete Edition, there is no way to partially uninstall either. Yet, my experience was different – if I do not want futuristic features, simply avoiding Omega Co. plants or the Academies will do the trick, but seems like it is not in your case. I suggest maybe double check whether you accidentally plopped any futuristic building; if not, then I cannot be of much help here.

Have a nice session.

You shouldn't be getting any futuristic content as long as you don't use them. That includes the newer water tower. If you plop futuristic buildings, the road will get futurized and all other buildings along that road.