How to block any app from being installed? My mom has an android tablet and she doesn't even know what a site is, her e-mail, or even the password of her facebook account. She's an average user. The internet is facebook in her eyes. She has no need for apps or anything, it's just, only, precisely facebook. All is facebook.

How can I completely disable installing apps in android so they can't be installed in any way or form, I mean, not by URL, not by clicking, not by app store, not even by another program. Just disable the entire thing altogether.

Doesn't need to be safe, have locks, passwords etc. I'm not trying to secure the device from intruders, attackers or robbers, I'm trying to secure it from its very own user.

Please help. I just had to remove two anti-virus apps that were opening adware pages when she tried to open facebook lite, and that's obviously a no-go, apps which she "had not installed." Clearly there's some voodoo going on here for them to just popup, or maybe she just did it unknowingly, but none of that's relevant. How can you just disable installs so this couldn't ever happen again?
First, disable the primary source of installation. On most devices, it is the Google Play Store but your device might have been shipped with a non-Google market app as well. So go to Settings &rarr; Apps &rarr; (three-dots line &rarr; Show System) or (All Apps) &rarr; your market app &rarr; Disable. 

Second, disable the Package Installer system app. This provides the GUI for sideloading. So if you disable it and attempt to install an APK (through a file manager, by drive-by download, or when pushed by a third-party app) you would definitely fail. At this point, it is irrelevant whether Unknown Sources is disabled or not, but you may keep the latter disabled from Settings &rarr; Security. 

Third, an app can still be sideloaded through command-line, preferably through ADB. Keep Developer options disabled and hidden (default). This is untested but revoking executable permissions (requires root access) from pm binary for user 2000 or for every user might help. This however, may cause device instability so take backup from a custom recovery before you proceed with this step. 

Your user would be fine with the first two steps. It's not like they would deliberately try to find a way to install an app if they are not tech-savvy already. 



Alternatively, in your particular situation you can use an ad-blocker app. The best ones require root access though. 

Or, why not use a kiosk-mode app? Limit your mother's activities to certain set of apps she should use. That's an another option on the table. 

Choose wisely.