Removing a managed server from XenCenter stops all managing and monitoring activities for that server. It does not affect the activities running on the server itself or remove any VMs installed on it. Removing a server simply breaks the connection between XenCenter and the server and its VMs, and removes all traces of it from XenCenter.
To remove a server, select it in the Resources pane, and, in the Server menu, click Remove from XenCenter.
To put a server that you removed back into the list of managed resources, you need to add it again to XenCenter in the same way as the first time you connected to it; see Add a new server.