I sincerely doubt that pilots decide "when to eject a passenger". Mostly it would be the cabin crew: the flight attendants are 100% in charge of flight safety, and they would be managing relationships with passengers, and they would be the ones to make the call. It would ultimately be them calling some kind of law enforcement. If an Air Marshal is onboard already, obviously they would be on the front line as well.

Furthermore, the concept of "ejecting a passenger" from a flight would mostly not be something you do while in the air, unless you're nuts. Ejecting a passenger is either done before takeoff, or your crew decides to divert the flight, or continue to the destination and have law enforcement waiting on the tarmac.

Naturally, pilots get involved when it's a question of where to fly the plane and when to divert, but ultimately the cabin crew is also involved in those decisions about problem passengers.