All companies will have to give their staff some form of training in ‘equality and diversity’. However, that doesn’t eliminate these experiences. Presumably this is an issue with the psychology of individual staff members rather than a corporate problem.

I would just feedback directly but politely to the staff member at the time, effectively providing some on the spot retraining, ideally in front of other customers. That may actually go a fair way towards at least curbing future outward expressions of inner prejudice.