You mean releasing him? That means they have to pay out whatever guaranteed money is left on his contract, which would probably be a disaster for their salary cap, and they also wouldn't be getting anything for him.
The proper solution is trading him if he is deadset on not playing. Green Bay rightfully is holding out for a major offer as they think he is worth however many first round picks. Other teams rightfully are probably lowballing them at this point, I'm not sure I've seen a verified trade offer Green Bay has actually declined. They probably think he will play for them eventually and not actually sit out the entire season.
This is a consequence of the increase in player power in the NFL and NBA. The NFL isn't quite on the level of the NBA, where it seems we've got to the point where players are signing a contract with the league instead of the team. The most prominent recent example is Damian Lillard, who recently signed a 4 year max contract extension with Portland before this past season and last week said that he is "undecided" on his future.