If you want to delete a plan, click on the gear wheel in the basic settings to open the configuration options and choose Delete Plan:
As a plan includes several important information such as versions, policies and change logs, you will have to confirm, that you really want to delete the plan including all its elements.
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The button to delete the plan will only be enabled if you enter the correct name of the plan in question:
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You cannot delete a plan that has already been locked. Locked plans will be deleted only if the complete organization is deleted.