Have you raised a complaint with the regulatory body they are registered to?
That also works well.
The only alternative to court is Consumer Disputes Board but their resolutions are just recommendations & Finnair has a long history of ignoring them so spending 2-3 years there seems like waste of time.
I generally go direct to the airline these days but if I get pushback beyond what I’m willing to deal with myself, I’ll use one of those services.
[1] Probably the majority of businesses does. But shady used car dealers etc. and Finnair don't.