We always advise to archive a client rather than delete them as you don’t pay for archived clients and this was you save all your proof of work and registers, however if you delete you will never get that information back.
To archive go to Company > Details and scroll down to put a date into company secretarial archive and press save.
To delete (not recommended) you must archive first, once you have put the archive date in you go to Company > Details and then click delete. If you don’t archive first it will not delete all the people connected to the company.