I will disagree in theory but agree in application. You say that it would seem unjust to removes a citizen who plays a role from society and that criminals belong to this category. I would say that, while unreformed, criminals play no rule in a just city. Thereby, permanent or temporary removal, until reformed, would not be unjust to the city; instead it would be unjust to allow criminals to remain in society. Moreover, justice is to give what is due, and what is due to criminal is exile according to Socrates. For a Christian it would be punishment in hopes of reform.