You can download them again from the cloud. Downloading them again does not require a passcode on 7.x. You can password-protect individual apps on jailbroken devices, but there is no way to password-protect them on a stock phone. You just have to use the lock screen passcode lock.
For what reason? Protect them from what? Do you mean hide them so no one can see your purchased list of apps? You can't hide your purchased list. You can restrict anyone buying or downloading apps in settings, but they could still see the purchased list. And doing that would be trouble for you. Every time you needed to download an app, you'd have to unrestrict it in settings. Did that answer your question?
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