Define fixed quantity per product (or category) and leave that product undefined, it won’t get any value.
If that product doesn’t need to be controlled by anything on this plugin, you can disbale it from Advanced >> Disable plugin by product >> and then enter relative URL for that product.
If you have too many products and you only want that product to be excluded (i.e. you can’t define the fixed quantities on all other quantities), just define fixed globally (plugin settings), and then on that particular product, enter 1,2,3,4, etc… (all possible values), and this will overwrite the global values (defined on plugin main tab).
I hope this helps, if anything else is further needed, please let me know.