If you double click on a client, chose "Edit" in the context menu or if you use the graphical button "Edit" in your client manager you will open the dialog "Client data". In this view there are multiple tabs available.
You can activate and deactivate the clients. If you have active projects, but an inactive client, it will still appear in gray font in your project manager. You should archive or finish all your projects and then deactivate the client in order to make the client invisible in your project manager.