if your enemy has no resources and his hand is full,
The only way my opponent's hand could be full is if I played a nightmare this turn. If one has a full hand they will be forced to discard a card at the end of the turn.
he will keep having to discard stuff that would otherwise be useful
Assuming you originally meant a 7 card hand when you said full, let me create a hypothetical situation.
Turn 1: I end my turn with 7 cards. My opponent plays nightmare on his turn.
Turn 2: I now have 8 cards and discard the useless card he nightmared.
Turn 3: I have 8 cards, all of them ones that I put in my deck and I must discard something useful if I am unable to play.
Now let's see what happens if my opponent does not play nightmare.
Turn 1: I end my turn with 7 cards.
Turn 2: I have 8 cards, all of them ones that I put in my deck and I must discard something useful if I am unable to play.
In the situation you described nightmare is either unable to be played or not worth using.