How can I disconnect a site from an UpdraftCentral dashboard?

A site can be disconnected from either end. If you have access to the UpdraftCentral dashboard, then you click on the site’s menu, and choose the ‘Remove site’ option. The connection will then be destroyed (specifically, the encryption keys used to make the encryption are deleted). If you have access to the site that is being controlled, then you should go to UpdraftPlus, and to the ‘Advanced’ tab, and delete the encryption key for the connection.