As synaptics is no longer available for controlling touchpad, we have to use xinput instead.
First, use command : xinput list to display all input devices and find out touchpad’s number.
In my system the result is:

⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ Logitech M215 2nd Gen id=10 [slave pointer (2)]
⎜ ↳ Logitech M280/320/275 id=11 [slave pointer (2)]
⎜ ↳ Logitech M280/320/275 id=12 [slave pointer (2)]
⎜ ↳ Logitech M280/320/275 id=13 [slave pointer (2)]
⎜ ↳ Logitech M280/320/275 id=14 [slave pointer (2)]
⎜ ↳ SynPS/2 Synaptics TouchPad id=17 [slave pointer (2)]
⎜ ↳ TPPS/2 IBM TrackPoint id=18 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ Power Button id=6 [slave keyboard (3)]
↳ Video Bus id=7 [slave keyboard (3)]
↳ Video Bus id=8 [slave keyboard (3)]
↳ Power Button id=9 [slave keyboard (3)]
↳ Integrated Camera id=15 [slave keyboard (3)]
↳ AT Translated Set 2 keyboard id=16 [slave keyboard (3)]
↳ ThinkPad Extra Buttons id=19 [slave keyboard (3)]

As displayed, my touchpad’s id number is 17. Next, just disable it using the following command:
xinput --disable 17

search result talks about an “input method selector” in preference menu which was not found on my system.
solution:
sudo dnf install imsettings-lxde

then you can find “input method selector” in preference menu.