I don't know if you can delete a service account (you'd have to contact SE directly for that), but they won't automatically apply it to an account, you have to select it yourself which one you want to enter the expansion code on, then it'll be added. So unless you select the wrong one, it won't be a problem

(Many people have multiple active service accounts so they can't/won't just randomly guess which to put it on, they have no idea and will leave it to us - at least they always have in the past so I can't see them changing that now)