Perhaps there could be a popup notification that asks if you would like to be auto assigned to a clan? This way it prevents people joining clans that they don't want to when they start a new account.
Presumably you don't disable the option to leave the clan. Alts can just leave as they choose. The key is to get players in clans, its then simple for those that don't want to be there/are alts to leave. Auto clan placement could very well be the best thing you could do for player retention. The only question is really where to place them and how to staff those clans.