You can not Delete an Account Plan that is assigned to one or more Customers. Only Plans that are not in use (not assigned to Customer Accounts) can be Deleted.
To Delete an Account Plan please follow the steps below:
Click the [Account Plan] > [Plans] menu button
Identify the Plan you would like to Delete and click the button
Review the Delete Plan conformation message and click the [Delete Plan!] button to permanently Delete the Account Plan