Social Snowball allows you to create affiliate programs that only apply to certain products. Here's how to do it:
First, head to the Programs tab, and click the edit icon of the program you wish to exclude products from.
Scroll to advanced program settings. Change the Applies to field from all products to specific products.
Next, specify which products you want the affiliate program to apply to. Discount codes that are generated for this program will only work on the products you choose here.
Now, hit publish and your changes will be live!