Close your account. You may not be able to cancel the number, but you can close your account. The number will still belong to the entity for which it was opened, and it can be reopened at some point in the future should the need arise. To close your account, write to the IRS at: Internal Revenue Service, Cincinnati, Ohio 45999 and tell them why you want to close your account.
Include the copy of the EIN assignment notice if you still have it. You will need to include the complete legal name of the business or organization, the EIN, and the business address.
File appropriate paperwork with your state. If you were issued a separate tax ID number from your state, you will need to go to your state's tax department and file the paperwork they require to cancel your account. Some states have a slip in the tax account booklet that can be filled out, while others have a notification form available on the tax department website that needs to be filled out and sent in.
File your taxes. Before you write to the IRS or your state to close your account, file your tax returns. Even if you don't own any tax, doing so will ensure that the account is clear before you close it.