The transfer window has remained open well past the start of the season for donkeys.
With regards to Isak, Newcastle would have my full respect if they told him to not bother reporting for training or games, and just let him fester for the remainder of his contract. Stick him on gardening leave and ruin his career. The complete malcontent players have these days and the amount of power they've been afforded has clearly gone too far.
Does he deserve to have his career ruined? Probably not but a club is going to have to take a stand at some point otherwise they may as well pack in contracts, they wont even get full value for him now, and you can be sure he won't be giving back whatever his nice signing bonus was for the contract extension he signed.