Unfortunately the pilots and crew are limited in what they can do to restrain a passenger thus they just turn around per the airlines policy and the police take him away.
I have a better suggestion if the pilots were allowed to do it but they are not now. A discreet tazer to the neck and a suggestion to walk forward to the jump seat after zip ties. Place several diaper pads on the jump seat and tell him to sit there quietly and tied up. Make it plain you will Tazer him again if you get verbally abusive and really want to do this and then duct tape his mouth.
Land in Rome and tell the local authorities to take care of this.
Or the Russian solution, “9x19mm Parabellum,” the standard side arm of the Russian Military today. The majority of those Russian civilian pilots are ex-military, they know how to shoot bad guys.
How many airplane hijackings have read about every in Russia?