Your Android need not to have root access for truly disabling an app, if you've version 4.4.x or above. All you need is adb setup in PC and USB debugging enabled in a non-rooted device or a terminal emulator app for a rooted device (you can use adb too).
If you check Package Manger's (pm) usage, you would see
pm block [--user USER_ID] PACKAGE_OR_COMPONENT") pm unblock [--user USER_ID] PACKAGE_OR_COMPONENT")
For Lollipop, it would be
pm hide [--user USER_ID] PACKAGE_OR_COMPONENT") pm unhide [--user USER_ID] PACKAGE_OR_COMPONENT")
In order to block or hide a package (it is safe), simply do
pm block PACKAGE # for KitKat
pm hide PACKAGE # for Lollipop
To unblock or unhide the package, do
pm unblock PACKAGE #for KitKat
pm unhide PACKAGE # for Lollipop
PACKAGE → package name of an app. To know the package name of an app:
- For KitKat and below, follow View app's full package name?
- For Lollipop, open Settings → Apps → All apps → your app → click here
The function behind hide has the following comment inside the source code
Puts the package in a hidden state, which is almost like an uninstalled state, making the package unavailable, but it doesn't remove the data or the actual package file. Application can be unhidden by either resetting the hidden state or by installing it
Similar commenting is done for block here.
In order to verify the claim, you can use some system services such as meminfo, procstats and activity using the dumpsys tool or even list all the processes using ps. You won't find an active presence of the blocked/hidden app.
The same goes for a lot of system apps disabled using GUI or pm disable but not for every app since even a disabled app can receive broadcasts it has registered for, which can only be done if it is loaded into the memory. Nevertheless, it cannot act on its own, neither can it be executed by any other app.
I've argued some of the differences between hide/block and disable on my question pm hide VS pm disable -- the identity crisis. It provides only supplementary info to this answer so you may skip it.