An Admin Charge is setup in the system like a Product; the Credit Account Wizard will show all products, unless the Admin tick box has been used, this field on a product is a filter mechanism.
Products associated with a product type which have Admin enabled are excluded from normal product lookups; products set as Admin will normally be things like charges, or other items that the customer doesn't actually purchase.
Therefore, ticking the Admin box on a product type is the way to exclude Admin Charges from things like the Credit Account Wizard.