In order to kill a main motion, what action can be taken?

To kill a main motion effectively, postponing it indefinitely is the action that achieves this goal. When a motion is postponed indefinitely, it is essentially set aside without any possibility for further consideration or debate in the current session. This action is often used when members wish to express their disapproval of the main motion without having to vote against it directly.

This option allows for a clear decision to be made regarding the future of the motion, providing an official way to prevent its discussion or a vote from taking place. In meetings governed by parliamentary procedure, motions can be manipulated in various ways, but postponing indefinitely ensures that the main motion will not be revisited.

Other potential actions like amending the main motion merely change its content, withdrawing the motion removes it from consideration but does not necessarily "kill" it in a fixed manner, and tabling the motion might simply delay it rather than prevent it from being further discussed in the future. Each of those other actions maintains some level of potential for the motion to be brought back up later, which is not the case with postponing it indefinitely.