If you want to delete a project plan, activate the data view Project plans, select the project plan and open the context menu with a click of the right mouse button. Select the menu item Delete project plan. You will see a message box where you can decide whether you delete the plan only, or delete the project plan including all work items or cancel the operation.
If you want to keep the work items, select the option Delete plan only. After the delete operation these work items will not be attached to any task.