If you have Supplier User you no longer wants to see your collection - you can Deactivate him.
Simply mouse over the supplier row and click the blue edit icon - or double-click on the table row.
When in edit mode, click the 'Select brand' drop-down and remove the brand assigned to the supplier.
Now this Supplier User can no longer see anything in your collection.
You CANNOT delete a supplier or a supplier user. Instead, you can deactivate a supplier or you can remove all brands from a given supplier user as described above.