If you have tried dealing with them and have had no success, maybe a small claims court action, issued on line will do the trick. There is now plenty of case law, to quote. Then if they ignore a court paper, they will lose by default, and have to pay or you can send the bailiffs in.
Also have a look here, for some advice.
I have an outstanding claim against Norwegian, but as they don’t have aUK operational address, ( until today), i have complained to CAA, who can also open a case for you.
Don’t give up as they want you too. Airlines that treat customers like this, don’t deserve customers.