So, this is the reply from Iberia.
Thank you for your notification regarding flight IBXXXX of 27/10/2018.
We accept all credit cards at Iberia. However, in certain circumstances, payment may not be made due to restrictions in the country, either for customs or security reasons as it was your case. It was your bank who waved it away as fraud control, so please contact them directly.
Kind regards,
Iberia Customer Services
Completely (deliberately?) missed the point. Phoned Customer Services and they said 'We don't deal with complaint by phone' and hung up before I could say anything else.
This seems to be the modern way - make it so difficult for customers to complain that they just give up.