Do I really have any other choice? You don't seem to be offering any other solution except to delete the giveaway. 🤦
I guess you can go ahead and delete the giveaway.

Hi,

Due to the rule of this web, I have to delete the giveaway.

Please, do not say this to the winners. Nowhere it's written that if you can't provide a working key, the giveaway must be deleted. It's written in the FAQ:

• You can acquire a replacement gift or key for the same game, which the winner is able to activate in their region.
• You can accept the giveaway as being "Not Received" by the winner.
• You can submit a ticket to have the giveaway deleted. In order for us to delete the giveaway, the winner will need to post a comment on the giveaway saying they agree to have it removed.

Giveaway deletion is last option, not the only one.