If you want to exclude a certain supplier from purchasing for a customer, there are 2 ways this can be set up.
Company Basis
- Go into Company Record > Product & Pricing tab
 - There is a box called Suppliers Excluded from Pricing Calculations
 - Click Add Supplier
 - Select the Supplier from the drop-down
 - Click Add
 
Location Basis
- Go into Company Record and into the Location you wish to restrict
 - There is a box called Suppliers Excluded from Pricing Calculations
 - Click Add Supplier
 - Select the Supplier from the drop-down
 - Click Add
 
                 