Hi,
Regrettably, there is no such a "button" or setting in Kentico
CMS however, you can use the
app_offline.htm file and place it to the root of your web site to turn it off for maintenance.
Or, within the content tree in CMS Desk - you can set an URL redirection for a section of your site so the users will be redirected elsewhere.
Best regards,
Juraj Ondrus