There is nothing he can do and there's certainly nothing he can do but play football once the window has closed.
Sancho didn't play bad on purpose, and different situation since (at least that's what I remember) he said he was always gonna be respectful of Dortmund's decision. Just a bad spell for a young player imo.
Kane knew he was signing a SIX YEAR EXTENSION at fecking Tottenham. What did he expect to win there exactly, a League Cup here and there? It was a braindead decision on his part if he wasn't dead set on being a one-club man. He also knew very well how difficult his chairman can be during transfer discussions, so it's not like something changed on that part either in the meantime. He made a mistake and is paying for it now, but staying away from pre-season training is just childish behaviour. Make your stance know and be a professional about it, it's not in your hands. Not sure what he's trying to achieve with this, Levy knows he won't hold out once the season starts / window closes, and he's not exactly gaining goodwill from Levy either with this decision.