Well, to handle this problem we need to take care of following mentioned things.
1) Uncheck “Allow installation from untrusted sources”
If this is unchecked, 99 percent of application’s from unknown sources will be blocked, Still there is a possibility that some applications can make it through to your phones, I’ll discuss that below.
2) Change play store settings (Auto update off)
Set auto update to off, this will prevent auto updates of unnecessary applications.
After following above steps, still if applications are getting installed, It means that there is something wrong with your phone, Follow below steps to fix the issue.
Search for system application “King User” in your phone, This is the application that might be causing issue. You need to uninstall this application, and for this root permissions are required.
Root your phone with help of King O root, (Connect your phone through usb cable, and install King O root on system, and ROOT your phone)
Then by using “Uinstall” app, uninstall the “King User” application and other unnecessary applications.
Hopefully your problem will be resolved. For any queries put your comments in comments section.