To register a complaint against a bank or other financial entity in Spain, the process is as follows:
The first thing you must do is to register a written complaint with the customer service department (servicio al cliente) of the institution in question. Be sure that you leave one copy with the company and have another stamped with the date received. In the case that your complaint is not resolved to your satisfaction, you will need this documentation later.
Before you can proceed any further two months must pass either without a response to your complaint, or without a satisfactory response.
At this point, you can submit your complaint, and all the documentation surrounding it, to the Banco de España. This entity's decisions are not legally binding, however, banks usually adhere to its rulings as they are well aware that the Banco de España is highly regarded in a court of law.
Spanish financial consumer associations are also available to help resolve difficulties between clients and service providers.