If you can't get a response online, try to find their legal entity in Spain (there should be one) and send a letter to the ET Spanish legal entity using a delivery service that requires a signature. Be brief and direct and clearly state that you are claiming under EC261/2004, and explain why your claim is valid. Include printed copies of email that they have not responded to. Give them a fixed number of days to respond (2-3 weeks) after which time you will pursue legal action to recover costs under EC261/2004.
Good luck!