[Edited Aug 07]
There is a option to set a maximum discount on a stock card. This will override any other discounts set in accounts even if the staff member is authorised to manually override a discount.
Go to 'StockCard', select 'Extra Options', tick 'Max Discount', then put in the value of the maximum discount allowed on this stock item, e.g. if you do not want any discount to apply to a sale item, then set Max discount to 'zero'.
This option can be used on sale items for an account person who would normally get 10% discount.