The three articles attached from the North Carolina Realtors® help explain the potential issues that could arise from excluding another agent or prospective buyer from being shown a property that is listed by a Realtor®.  

In addition to the guidance provided in the articles, Canopy MLS advises that if the seller is intent on  disallowing a certain individual from being able to show the property, then the listing agent should handle the appointments for the listing.  The listing agent should provide instructions to ShowingTime that all appointments must be confirmed with the Listing Agent. 

Additionally, do not include in the listing in the MLS any statements or remarks  identifying the individual who is not allowed to show to avoid the perception of a boycott of that individual or firm.