The fastest way to make certain Menus, Menu Groups, Menu Items or Modifier Options not eligible for discount is via the Advanced Properties Editors under the Menus Section of the Toast backend.
From this editor page, the Show/Hide drop down menu can be used to add a column for "Can be discounted?"
This will add a column to the table below which allows the user to change the answer between Yes and No.
In the example above - the user has opted to make alcohol not discountable unlike the rest of the menu. This is the most common example of this in the restaurant industry.
To finalize changes, don't forget to save and publish!