You have to use a UI mask in Reshade. There are tutorials on how to do it online if you Google it, essentially you are creating a plain white background the same resolution you game in, and then using an editing tool to paint black all the areas you don't want effects in Reshade to affect.

You then replace the default background with your custom one and activate the mod, with UIMASK_TOP at the top of the load order, and UI_BOTTOM at the bottom of the load order (or top and bottom of just the effect you don't want applied.)