Hey there Andrew!
Genevieve from Shopify here :)
As it's possible to set overlapping collections (i.e. to have products belong to multiple collections) you could create a collection that includes all product except those you don't want the discount to apply to. You can then apply the discount to that collection only.
You can hide this collection from the storefront if you like, by turning the Online Store visibility off for the collection. That way you can use it for organising things like discounts in your store admin, but customers will not see it.
Hope that helps!