A client can only be deleted if no documents whatsoever are issued towards that client. If you can't remove all documents from a client, but you still want to remove him/her, you can mark the client as Inactive.
Actions -> Edit on that specific client. Untick Active. Save Changes.
Inactive clients won't show up anywhere on your system, but you can mark them as active again in future.
Updated: Sun, 25 February 2018