The Complaints form system is very useful, so how does it work?
The complaints form must be available in every establishment in Spain, including the police, hospitals, town halls etc. If you ask and are told that the establishment does not have a form then call the LOCAL POLICE. They should attend, demand the form, and if it is not supplied, issue an immediate fine and close the establishment.
The form, comprises of three pages on sensitised paper (two are copies of the first page). It is printed in Spanish and English. Complete the form. (It is debatable whether this can be done in English or of it must be in Spanish, but as it is printed in English too, then I would give it a go in English if you cannot speak Spanish).
Printed down the long, left hand side of each page, is an instruction as to which copy is for whom. One goes the establishment complained of, the other two are kept by the person complaining.
If after, I believe it is two working weeks (maybe ten days) you have not received satisfaction, take the copy, so marked, to the consumers office of the town hall. They will then pursue the matter.
PS I have asked for the form on a few occasions in a National Police Station on behalf of dissatisfied 'customers.' The results have been amazing and the person who had the grievance received satisfaction each time, there and (very quickly) then.