I know this is a very old post, but since I googled this specific problem and found this topic, I think it would be best to post here so others can find it.

The option to turn off dlc was moved to the options menu under gameplay, but you cannot access it unless you are on the main menu. You cannot switch them off during the game. Hope this helps.

Yeah this is where people get tripped up I think. Has to be on the actual main menu, before you load anything.

I like the Triss alternate outfit, and it matches her style from the first game when she lived in a city, but as many have said it doesn't make much sense for someone in hiding to dress so lavishly. Also I read it won't work in Blood and Wine, which is a bummer. So I put them all on pre-DLC.