If you have products that are not returnable, also called final sale items, all you need to do is to setup a return logic that does exactly that!
Go to: Settings -> Returns -> Create a new return logic
Login using a product attribute
Under Conditions you can set a logic similar to this one in case you have a product attribute such as "final-sale"
Login using a product SKU
Simply select "product SKU" and list out all SKU's comma separated in this logic
Logic using discount value
Using discount value you have two options:
% based -> 30%
$ based -> $50
Are you looking for other logics? contact us and we can make it happen for you ;)