Your first example isn't the best example for an ad hominem (in my opinion anyway). Because in "you're just paranoid" it could to be implied, that he is being paranoid about this subject in particular. An ad hominem would be to claim, that John is a paranoid person in overall and than to conclude, anything he is concerned about must be absurred.
It sounds more like a different fallacy, "appeal to the stone", because Steve seems to be inferring, that John is being paranoid, because his claim is absurred. However, Steve just inferres, that his claim is being absurred, he doesn't show how his claim is being absurred.