I don't think Casty is actually disruptive. He had one little heat-of-the-moment blow-up. It's not even true that he ever insisted on playing every game last year, it was something he said half in jest that Thomson decided was an admirable goal for him.
Kepler was more disruptive with his complaints about not starting, and while that has blown over, demoting him to full-on bench player might not go over well. But seems the only real path.
Problem is Kepler's results at the plate are just so poor. Casty's still got him beat against RHP by 50 OPS points this year. That doesn't mean going by the book/larger career sample and defense isn't still a good idea. But Thomson specifically called Nick an everyday guy just yesterday.