To unassign an ASIN from a product in Marketplace Connect, simply delete the ASIN in the relevant cell (delete and click elsewhere on grid).
Once the ASIN is deleted from the grid you need to change the category assigned for the product - it doesn't automatically reset.
You will need to change the category manually from the Category column by clicking on the current category value and changing it.
For further assistance, please reach out to marketplace-connect@shopify.com