without rooting the device and still be able to change build.prop, no need for adb nor developer tools
(after 7 hours trying and finally getting there, I won't care for my bad english)
in short: I found the right twrp for my device, then use latest odin to flash twrp on it. then from twrp advanced file manager copied build.prop from system to sdcard, restart, modify the copied build.prop, restart into twrp, rename original file (basically make backup), and copy back the modified file into system. then select it and use the button chmod 755 (or go in terminal and chmod 744 build.prop, note that only root can have write permissions or your device wont boot).
specific: (tested on samsung tab a t550 9.7" with marshmallow 6.0.1)
-google and download latest odin, unzip it.
-google twrp and your device, download it, I don't know if the latest twrp works for every device but at worst it won't but it wont harm your device.
-search how to restart your device in download mode, mine was pressing power+home+downvol for aprox 4 sec (while the device was powered off, obviously?).
-when the download screen appears, connect to pc and open odin, the device detected under com should be blue
-press the ap button and browse to select the twrp file you downloaded
-verify that re-partiotion is not checked under the options tab
-the device reboots normally, you need to power it off and start in twrp mode. in my case it was done by pressing power+home+upvol for aprox 4 sec
-in twrp, don't start in read mode (the first time it asks), you wont root but you will change a system file.
-in my case while using twrp, the touch detection was a bit off to the left and I needed to press harder so beware the annoying and damaging fat fingers
-press the mount button, select system so it's checked, and uncheck read mode if it's selected.
-go back (press home) and press the advanced button, then file manager.
-go to system, then select build.prop file
-press copy file, select a destination folder where any user can read it, I used sdcard (you can use the terminal to chmod the file copied but it is faster from windows)
-go back and press reboot, then system. it will reboot normally.
-conect the device to the pc and open the copied build.prop (notepad won't work use notepad++) and edit what you want, in my case I only added fw.max_users=1 below the last line. save it.
-if you could't open it because of permissions, first copy the file into desktop and then modify that file then copy it back and replace it.
-disconnect the device and restart into twrp
-again go to mount and check system, then go to advanced and then use the file manager to go to system and select build.prop
-rename it (add .bak at the end or whatever) so it's backed up
-browse to the modified build.prop and copy or move it to the system directory
-browse to the system directory and select the newly copied build.prop, then press the chmod 755 button and confim, as i said, even if you don't modify the original file, just changing permissions to grant write access to anyone besides root, will make your device hang on boot if that happens, restart by pressing power + downvol for some time. then immediately after it powers off press either power+home+upvol(twrp to change back whatever you did) or power+home+downvol(download mode, if all fails you can still flash the official image from odin, you just have to google your device and the android version then official image, make sure it's the right one)
-restart and it's done, no more guest or add user, and no rooting, altough from download you can flash a root image (if there is one for your device) or from twrp you can install supersu to become root, you can say this method is as difficult as rooting but I prefer to avoid rotting. twrp will be there but it won't get in the way of anything.
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