When players have set on to leave a club what the club can do is to work ways to convince him to stay, if force is used to keep the player against his will it will be chaos for the team.
He can disorganise the team play to frustrate the club and benching him won't stop him from earning his wages. Newcastle should release him for the money Liverpool are offering puts Newcastle united in large profit to what on a normal Alexander Isak is worth in the market.
If Newcastle united decides to punish him for his actions by not selling him until his contract expires they will be in the losing wing, because Isak will still sign for Liverpool as free agent to the overall perk of Liverpool.
But as a professional, especially as a professional player, he should act professionally. Newcastle have so far refused to let Isak go to Liverpool, and they have issued an official clarification. However, there will certainly be some negative thoughts about Isak's future performance after his desire to leave was not met. However, if his performance declines and he is deliberately trying to distance himself, that would be too risky.
On the other hand, Liverpool are also focused on other signings. They still want a new central defender for this transfer market, having previously signed Giovanni Leoni, a center-back from Parma. This is because the agreement with Marc Guehi is on.