Separate GUI from FTP Svc/"Quiesce" ability

Almost any time I need to do maintenance on the server, I'd like to be able to "quiesce" the server - not accept new connections (perhaps with an 'under maintenance' message?) but allow current transfers to complete. As it stands I have to stop the FTP service which also disables the GUI. I'd like to be able to change settings with the GUI (like replacing the certificate) without transfers going on.