I need to permanently disable the touchscreen input - Ubuntu 16.04

Try finding your touchscreen XID with xinput --list, then add the following command to your startup applications: xinput disable [touchscreen XID] (for example, if the XID is 9, then the command would be xinput disable 9).

The link shared by @Whiteboy works.

In file /usr/share/X11/xorg.conf.d/10-evdev.conf, changing the "evdev" to the "libinput" in "Driver" section of the touchscreen should do the trick.

So now it looks like this:

#
# Catch-all evdev loader for udev-based systems
# We don't simply match on any device since that also adds accelerometers
# and other devices that we don't really want to use. The list below
# matches everything but joysticks.

Section "InputClass"
Identifier "evdev pointer catchall"
MatchIsPointer "on"
MatchDevicePath "/dev/input/event*"
Driver "evdev"
EndSection

Section "InputClass"
Identifier "evdev keyboard catchall"
MatchIsKeyboard "on"
MatchDevicePath "/dev/input/event*"
Driver "evdev"
EndSection

Section "InputClass"
Identifier "evdev touchpad catchall"
MatchIsTouchpad "on"
MatchDevicePath "/dev/input/event*"
Driver "evdev"
EndSection

Section "InputClass"
Identifier "evdev tablet catchall"
MatchIsTablet "on"
MatchDevicePath "/dev/input/event*"
Driver "evdev"
EndSection

Section "InputClass"
Identifier "evdev touchscreen catchall"
MatchIsTouchscreen "on"
MatchDevicePath "/dev/input/event*"
Driver "libinput"
EndSection

This works in my Dell 5558 laptop.

I've spent a long time looking for a solution to a similar problem I've been having for my old Asus X200MA. I never could find a software solution that stuck so I eventually opened up the laptop and unplugged the cable for the touchscreen. (It was way out of warranty anyway.) It's not elegant, and it may or may not even be possible on some other laptops, but it worked for me.

This iFixit guide might help.

https://www.ifixit.com/Device/HP_Envy_17-3070NR

It looked like the guide on the top cover removal might contain the needed information. If, however, the screen control cable and the touch screen cable are bundled together then this method obviously won't help much.