To delete roles, find the person you want to remove roles from in the list, and click Edit client rights. You will then see what clients the person has rights for. To change this, click the client and unselect roles or click Remove all. You can also remove all rights for the person by clicking Remove all clients.
If you want to remove roles for multiple people for multiple clients, you can also use client delegation by file. Instructions on how to remove roles is shown when clicking this option.