You can control if a product can be discounted at the Till via the Stock Card. You can do this one by one or in bulk via Product Quick Fix, refer to Product Quick Fix Overview.

From the Stock Card

From the Product Edit window: 

Tip: The Product Edit window can be accessed from:
Product Maintenance, click Change.
Order Editing, click Stock Card.
Price Manager: Pricing Review, click Stock Card.
Label Queue, click Stock Card.

1. Check or uncheck the Discountable checkbox.2. Click OK to save.