Hi @DevoDog,
A scheme with no threat is not a legal target for a Thwart action, as Thwart specifically requires there to be threat present to remove. Therefore, you cannot take a Thwart action on a scheme that has no threat on it.
Similarly, you cannot take an Attack action on an enemy that has no health remaining (see “21095”).
There is an exception: your hero can declare a Thwart action if they are confused, or declare an Attack action if they are stunned.
Hope that helps!