At a restaurant, an employee saw I was a foreigner, gave me a disgusted look, and refused to take my reservation, saying they were fully booked.

Getting a disgusted look just because you are a foreigner can surely make you feel bad. However, everyone experiences things differently, and it would be hard to make an objective judgment about this-if the employee had a displeased look on their face or if that look was for you because you are a foreigner. As for the reservation, the restaurant might actually have been full. You can seek an apology from the restaurant for that person’s attitude, but whether you will get one is up to the restaurant. Maybe the only thing you can do is tell the restaurant manager what made you feel bad, and ask them to change for the better.