Spanoulis hasn't really violated his contract, he's just not coming back to the NBA. There's no such thing as voiding it for non-play or whatever; a guarantee is a guarantee. When Macijauskas came over and proved to not be worthy for the NBA, New Orleans bought him out to send him back to Europe.
So the question is whether Spanoulis will hold the contract over the Spurs' head, knowing they don't want to give him any of the money. The see-saw of negotiation is certainly tipped in the Spurs' direction, since they can prevent him from playing at all, but he can play hardball if he wishes. (For my money, he won't, but who knows.) I still wonder whether his contract can be cut from the salary roll, but don't think so, which means the luxury tax dump wasn't that great.