A few hours after the first email, I received this second one with a threat (again, translated by me; here is the original):

Mr. Cortell:

I can prove my identity and the defamatory content of the indicated page, as I said it has been notarized. I can also prove that this site is currently the subject of proceedings in the Courts of Barcelona, in which the prosecution has already ruled, appreciating criminal evidence. But I see no need to do so, since you have shown no willingness to cooperate (“I do not wish to receive further communications from you”) and your certainty of not being subjected to Spanish law enforcement, since you abhor it and live abroad.
So, consider yourself sued.
Best regards.