If I start the simulation with plane the plane takes off and flies away automatically.
How can I stop that ?
throttle is engaged automatically in SITL. that is normal
no, throttle is off.
And the plane is disarmed.
in SITL the throttle output minimum is 1100 by default, this means idle, not stopped motor. So your plane is taxiing on idle. Change it to lover value, or ignore and do a takeoff.