Here´s the link. www.bde.es/servicio/reclama/bancos.pdf
I found it on www.eyeonspain.com, or by putting "how do I complain about a Spanish bank" into your web browser.
If you are not satisfied with the solution, you can present the claim before the Bank of Spain:
Banco de España Servicio de reclamaciones Alcalá, 50 28014 Madrid Teléfonos 913 385 068 y 913 385 851
In both cases you need to enclose personal identification, identification of the branch where the problem has happened, explanation of the issue with related means of proof and the statement of the case not being under judicial affair.