score:3
You can use the EU261 complaint form to submit a complaint to the relevant National Competent Authority as given from the link at the bottom of the Air Passenger Rights page.
Note the following text on the bottom of the 'Instructions' part of the complaint form however:
Please note that the competent authorities of Member States cannot in general take binding decisions on airlines in respect of individual complaints. If you are still not satisfied with the airline's response, even following the answer from the competent authority, you will have to pursue the matter in Court or through alternative dispute resolution.
Ultimately, your final recourse is through a relevant court or dispute tribunal process, which will depend on the countries and airlines involved. That doesn't really answer your question, but there are 28 EU countries plus associated other countries which enforce EU261, so a full list of the processes is out of the scope of this site - the Law StackExchange may help however.