Aristides (/ˌærəˈstaɪdiːz/; Greek: Ἀριστείδης, Aristeides; 530–468 BC)
was an ancient Athenian statesman. Nicknamed "the Just", he flourished
in the early quarter of Athens' Classical period and is remembered for
his generalship in the Persian War. The ancient historian Herodotus cited him as "the best and most honourable man in ...
No, in this context "an Aristides" is an example of someone who probably should not have been ostracised. It could have been written "_ ... sometimes banished an unfortunate person, for his reputation of Justice;_".
In your alternative version "sometimes banished Aristides" means that they repeatedly banished Aristides.