My guess is that "barred" means that the banks were closed and no longer allowed to do business.
Student or Learner
According to definition 1 in this dictionary for the verb "bar":
"to put a bar or a set of bars in front of a door, window, etc., so that people cannot go in or out of it
So, "bar" should be used with "window", "door", etc, but not the entire building structures. But then there is this:
"THE European Union has never seen the like of the past eight days in Greece: barred banks, capital controls, the first IMF default by a developed country, the collapse of a multi-billion-euro bail-out, plans for a referendum that may hasten Greece’s ejection from the single currency, and the beggary of the people."
Is "barred banks" an error? Would "banks with barred doors" be better?
Used as an adjective as it in this sentence, it just means blocked or barricaded to prevent entry. Could be the doors, could be the windows, could be both - doesn't matter. It's closed and inaccessible.