I'm running kubuntu 12.04 i386 on lenovo thinkcenter.
There are no options for the touchpad under "Input Devices".
Is there anyway to turn off tap-to-click by editing some text file?
I think I had the same problem. No touchpad settings in systemsettings. But I don't remember how I fixed it :(
But I know that you can configure tapping with synaptiks.
Just start a terminal and type synaptiks or alt-f2 synaptiks.
Now in 2020, the Solaar application handles these functions for many Logitech keyboards including the K400. It should be in most distros, and the solaar package is in Ubuntu's default repositories in 16.04 and later.
Control Panel > Change Mouse Settings > Hardware choose the right Mouse driver (for me it was called HID-Compliant Mouse) > Properties > Change Settings > Driver > Disable
Hope it works!