Fire 'em All
The solution has already been demonstrated, in the US in 1981.
Give them a short period of time to report to work, after that don't bother - you no longer have a job.
Privatize if needed, but salaries adjusted to a reasonable level, with fired strikers allowed to apply to work for the new lower wages.
If unemployment is high in Spain, there is no reason to believe that a high percentage wouldn't face reality.
Short term pain for aviation, but long term gain for Spain, and by extension, the rest of Europe.