I personally know of a similar case of someone who left Spain over 3 years ago. They wanted to keep their account open for a couple of months after retuning to the UK, "just in case", but were told by their Spanish bank (Sabadell in this case) that it could only be closed in person, in the branch. As it was a joint account (despite being "either to sign") they were told both account holders would have to come in. When they explained this would not be possible, they were advised to simply "zero" their balance when they no longer required the account, and forget about it.
Three years later a former neighbour of theirs forwarded on to them a letter from a debt collection agency, trying to claim over €300 in charges on behalf of the bank.
My advice: ignore the charges / threatening letters and forget about it. Anyway, as far as I know, there is a 5 year statute of limitations on such debts, after which any such claims will expire.
Stupid situation, typically Spanish problem.