I am having a hard time finding an answer to this question for 18.04 LTS.
I used the following to find a list of options to configure.
gsettings list-keys org.gnome.desktop.peripherals.touchpad
It returned the following list:
touchpad send-events natural-scroll tap-to-click two-finger-scrolling-enabled left-handed click-method speed scroll-method tap-and-drag edge-scrolling-enabled disable-while-typing
Out of this list, which of these options can be configured to be "false" to disable horizontal scrolling? Samuel Cavazos' answer seems close to what needs to be done, but the horizontal option does not appear to be available in the current list above. His answer, which is not applicable in 18.04, is as follows:
gsettings set org.gnome.settings-daemon.peripherals.touchpad horiz-scroll-enabled "true"
Was this option renamed to scroll-method? If so, what should the terminal line be to disable horizontal scrolling?