If you can't select the dlc on the game properties, and then game is free and the dlc, also how they have made this then it might not be possible to undo it without help from game support.

Check other game with dlc's its a standard menu with steam, rest is each own game launcher's, example some Behesda games use that way to load DLC'S.


ps.
Don't expect much help with free game and dlc's. ( they can see it as all-in-one product then its add itself as free. ( free stuff will always be added by themself. ) this is why i doubt you can have a choise at all.

"because support isn´t working for me"
And you dont post what they said, it's like you think you have support as a bought game you dont. ( and will not help you then dlc is free. )

And its not your files, its game devs and you have a license to play it, many pc user forget that part alot.. ( so yes gane devs can do as they see fit. )

So you might have no choise at all. ( other then stop play it and tell game support, ho they ruin the game for you, belucky if they will change or hear on you. )

i'm not the problem here, this is you and that game devs that did it. ( even steam can't help you unless its made by valve, and gl contact game support for that game you talk about. )