Social Snowball allows you to create affiliate programs that only apply to certain products. Here's how to do it:
First, go to the Programs tab and click on the program from which you want to exclude products.
From within the program builder page, head to the Advanced Settings section. In the Discount codes apply to field, change All products to Specific collections or Specific products. An additional field will appear where you can select which products you want the affiliate codes to work on. Discount codes generated for this program will only work on the products you choose here.
Click Save & Exit once done to set your changes live!