I have been using Blesta for many years (I've been storing up my feature requests for years too - sorry!) and I know that this topic comes up from time to time, however, I would like to give what I believe is a strong case as to why it should be allowed to delete clients.
Firstly, I realise that it is not possible to delete clients if they have an invoice or service attached and I believe that the reason for this is for accounting purposes in particular geographic locations (one of them being the UK it would seem).
However, in the UK we also need to comply with Data Protection laws. This says that we must not retain personal data for longer than necessary. See here: https://ico.org.uk/for-organisations/guide-to-data-protection/principle-5-retention/
According to the above page, we are allowed to retain the data if required for tax returns and this will not be considered to be retained for longer than necessary. So far so good...but according to my research, HMRC says that you only need to keep your business income records (including sales invoices) for 5 years after the submission of the tax return: https://www.gov.uk/self-employed-records/how-long-to-keep-your-records
Therefore, my feeling is that UK businesses should be removing the client records after 5 years of them ceasing the relationship with the business, thereby complying with the data protection act that says that you must not retain personal data for longer than necessary. This is how I interpret the law and in my opinion this makes a much stronger argument for the necessity to be able to fully delete client records from Blesta.
Also submitted to: https://requests.blesta.com/topic/delete-client-for-data-protection-reasons (posted here for awareness)