The card reads that if a ship ends it activation within 1-3 of you, that you may exhaust one of the enemy ship's defense tokens.
What happens if one of the defense tokens is already exhausted? Could you cause it to be discarded?
Looking at the rule reference it always talks about the spending of defense tokens, it doesn't use the verb exhaust. Yet other cards are use the verb exhaust to use the card.
So ... this feels to me like an already exhausted defense token can't be discarded. I wouldn't think that FFG would accidentally mix the verbs. Still ... seems like something to explore to make sure.