Disable an account in Apple Mail?
There are few ways to do this.

My Preferred way is::

Move all mails and folders you want to keep to "On My Mac" location.

Then in Mail app open Preferences--Accounts.

Uncheck the "Enable this Account"

It is now deactivated, but you still have the copies of the emails in your "On my Mac" location.

Sorry, the instructions are for 10.9.5.

As you will see the account is still there but grayed out, thus you can reactivate at any time.