Fix Object in Gazebo Simulation after Spawning

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I am using Gazebo-11 with rospy (python) in ROS Noetic. After spawning an object (urdf) on a table with rospy, I want to fix it in the world frame, so it does not move any more.

PS: I know how to spawn a fixed object (by adding a fixed joint with the world frame to the urdf file before spawning it). However, here I want to fix it and unfix it on the fly in the simulation while it is running.

Is there a way to do so? Thanks.