Navigate to Return-Settings / General in your Rich-Returns dashboard.
There is a field for "Excluded product-tags" near the top of the page.
Add the product tag of the affected product exactly as you have written it in your Shopify store for the product.
One product-tag per line (no comma inbetween).
A product-tag consisting of multiple words goes on one line (Excluding items based on product tags: I have a tag with two words... how do I write it exactly?)
The product will show up as "Not eligible for return" for the customer during the return process.
Tip: To prepare the customer for this situation and reduce support inquiries it makes sense to clearly communicate any excluded products and categories on your own returns-policy page.