There's no need to keep any apps on your homescreen. If your manufacturer's default launcher won't let you hide or remove everything, install Nova Launcher instead. You could then hide all apps in the app drawer (except the one you want), remove all apps (except the one you want) from the homescreen (and reduce the number of homescreens to 1), and even remove the dock or remove the app drawer from it. And then lock the homescreen.
I'm sure you could do the same with Apex launcher, which as I recall is a little less obvious about how to unlock the homescreen.
It won't stop someone who knows their way around Android from finding other stuff, but it will hide it all from most people.