I wanted to say that they should not have that right, however, looking at GDPR, perhaps it's not forbidden after all. However, it's worth noting that it's not

I wanted to say that they should not have that right, however, looking at GDPR, perhaps it's not forbidden after all. However, it's worth noting that it's not just a single exceptional person - Belgium has accented letters in two of their three official languages and names with accents are reasonably common, so if you tried that, you would have to discard many customers, and also those customers would be overwhelmingly from the french-speaking part of the country so that might be treated as explicit discrimination targeting the french-speaking minority community.