Could the non-smoking sign still be turned off in modern airplanes? Yesterday, I flew on Germanwings 4U2379 with an Airbus A319. The plane's interior looked pretty modern, and while I can't find a construction year online, I'm pretty sure it was built way after the total smoking ban. 

This plane still had illuminated non-smoking signs though and I was wondering if the crew would actually be able to turn these off during flight if they wanted to. Is there any information about that?

This question shows that some planes do not even have a non-smoking sign anymore, but it does not elaborate on the mechanics of existing signs.
It would appear that the aircraft you flew was D-AGWR, a A319 with MSN (Manufacturer Serial Number) 4285 delivered in April 2010 (hence fairly new).



There is a switch at the bottom of the overhead panel, next to the seat belt sign, slightly right of the middle. Flip it and they will extinguish. The functionality is a bit more complicated than it would appear, since it feeds into the central aircraft electronics. 



Olivier Cleynen at Wikicommons

Out of interest, this button no longer exists on the Boeing B787, suggesting that it is on the way out. Only the seat belt option remains.

The slots for no smoking are being increasingly 'redeveloped' into a sign for electronic devices instead on some types. They just put a no smoking sticker on the seat in front of you to fulfil the same function. I do not know if they cockpit text has been modified on the aircraft that undergo this modification.



Source