You can delete a plan via the Admin Console UI, or API (https://docs.recurly.com/docs/plans#section-updating-and-deleting-plans).

Once a plan is deleted the hosted page URL will return a 404 page not found message so new customers can not sign-up for the plan and the plan details will no longer be accessible via the Admin Console. Existing subscribers will continue to renew, and you will still have access to the subscriber data for this plan in your exports, however, you will no longer have the option to add new users to the plan.