NumLock is completely useless, and I want to disable it completely on Linux

Well for your first question, you can remap it to nothing with xmodmap:

xmodmap -e "keycode # = """

where # is the scan code of NumLock. You can find the scan code using xev. Instead of nothing you can remap it to any key you'd like.

I noticed this was tagged "Gnome", however for those who ended up here and are using KDE, it is really easy to make the numpad always print numbers.

In System settings > Hardware > Input devices > Keyboard > Advanced

Check "Configure keyboard options" then check "Miscellanous compatibilty options" > "Numeric keypad always enter digits (as in Mac OS)"

Click apply and it works directly.

"Miscellanous compatibilty options" > "Toggle PonterKeys with Shift + NumLock" Will turn on/off mouse emulation by Shift + NumLock, however this is off by default.

For Ubuntu 16.04 - xmodmap is not a permanent solution. Instead, install "Gnome Tweak Tools", navigate to Typing-->Miscellaneous Compatibility Options, and select "Numeric keypad keys always enter digits".