I've been asked a couple of times as to how a client can stop customers using their store, whilst they can still access it and manage stock etc. A feature within OpenCart is "Maintenance Mode" which shows a message to customers who try to visit the site, but still allows logged in admins to browse the site (and admin) as normal.
To find this setting, log into admin and go to System > Settings and click on the Server tab. From there simply change "Maintenance Mode" to "Yes".
If you don't like the message that shows on the site, or you still wish for people to be able to browse the site just not be able to use the shopping cart (and that includes Search Engines - as if you block out people you'll block our robots and therefore may lose your ranking in Google) then you can buy a $10 plugin for holiday closures.
Alternatively you could get your web developer to put up a holding page/take the site down but this would take some time/work if you still needed to be able to access the admin.