Disable discount calculation for specific item groups

I would like to disable the Discount calculation for “Services” item group. For example, if the standard selling rate for item “Cycle Repair” is defined as 100 USD in the Item master and if I set the rate as 50 USD in a Sales Invoice, DISCOUNT should NOT be calculated for that item. How can I achieve this?

Pricing Rule will NOT work as it wouldn’t allow editing the item’s rate with zero discount.