First step, create a list. You will do that by going to the search page:
From here, you will click on “New Search” and under the “SKUs” column, you will be able to paste the correct SKU that you need blocked. It will look like this:
Click the “Search” bar at the bottom of the page and it will bring you to the list of SKU’s. Click “Save as new” and label something like “Blocked SKU’s”. Once you have saved and labeled, return to the “Channels” page.
Under the “Blocked Search Filters”, you will put the name of your list you just created.
Click “Save”. It is crucial to select “Sync Inventory” at the top of the page. This will update all information you have just created!