In clientbase you can set clients as inactive so you can then toggle your client list to show or not show the inactive clients.

To do this select the client and click edit / set client as inactive.

You can then toggle the list through View / Inactive clients and then set to show or hide.

If you need to reactivate a client you can do this by setting all clients to show, then selecting the one in question and going to edit / set client as active.