Handing back the keys to the bank is known as "Dacion en pago" in Spain but this is not possible to do on a property which is in negative equity.
And most banks won't entertain dacion en pago anyway as the last thing they want at the moment is more properties on their books.
There was a reported case in a UK newspaper about a Spanish bank chasing a couple for a mortgage debt in the UK. It's done via a European Enforcement Order.
Search for that in the search box on the left and you'll see a few discussions and blog posts about it.
Best of luck.
Justin