Of course you can not show an occupied property unless the tenants are willing and even then the owner/rental agent has to use a great deal of discretion so as not to disturb the tenants. As for refusing to show an apartment, this sort of conduct could be considered discriminatory in the US and illegal. During the days of segregation it was common for owners and real estate agents to avoid showing property to potntial buyers on the basis of color or by a manner of speech that was interpreted to be other than Anglo-Saxon. There are now strict laws to prevent this type of behavior. There may not be equal protections in Buenos Ares. If not, there should be.