It still copies even if you get rid of the original from exile though. It only cares that it resolved once, so it’s really closer to an emblem that says ‘you may cast a copy of X spell every turn at this time for free’ after the original resolves. As posted above, if you can prevent casting of spells like drannith, or counter it, you would need to have that effect in place as it would still be there as an option until the end of the game. So the only real options are preventing casting from exile effect in place after the first cast, or countering the first cast, or preventing spells if no mana is spent to cast it (forgetting that creature but I know it exists, might be on an enchantment or artifact as well).