Yeah, once he signs the offer then he can't be signed and traded. The only thing the Spurs could do is try to force Utah to rescind the offer sheet, but DBrown would have to agree to that too.
Maybe the Spurs just want to screw with the Jazz. Of course, the Spurs still have about $1.7 mil in cap flexibility they could use. Not sure how much cap room the Jazz have left, but if the Spurs and Jazz were interested in the same player, it might be worth the Spurs' time to drag out Devin's departure while pursuing whoever else they want.