In the English-language media, I have read and heard stuff that has moved away from arguing issues and is at times abusive and ugly. If someone makes a sweeping generalisation about Greeks, simply asking 'All of them?' or something similar might make the person stop and think. You could suggest that their argument would be more effective if they kept it to criticism and left the abuse out. Ask them if they'd feel comfortable saying the same things with different peoples/ethnicities/religions.