If someone leaves your band, you usually want to remove their access to your account; and if you replace them with a new band member, you might need to remove them to fit the new member into the user limit for your account.
However, it's not a good idea to delete the old username. BandHelper won't let you delete users that are linked to past transactions and events, and if you remove the user from those items first, your past data would be incorrect.
Instead, you can simply uncheck the Active box on the user edit page. This will prevent the user from logging in or syncing, and will stop that user from counting against your account limit, but all your past data will remain intact.