In the past, you could tell steam to not install updates, but they took that away so now updates are forced. :/
As for reverting, I don't think you can anymore. Even if you re-download the game, it'll be the new version.
Indeed I remember when Steam did this folks where ****ed and still are to this day....SIGH...
if you are all dying to cheat, you could revert back to the previous version which was before the full game launch.
Go to game properties in library, go to Betas tab, under Select the beta choose: 3.0.77.309 and download the trainer from May. Or you could not do that and experiment with cosmos, which still works
[Edited by Arminaz, 9/14/2017 9:27:40 PM]
Unfortunately this would mean that only the first act is available, nor the 2 new presets, not the undead races. Saves would also be uncompatible with the release version.
We'll just have to wait patiently for the CH crew to update it again. Hopefully it's not much work, as that would really suck so shortly after getting one out for us.