How To Disable Mouse Acceleration With libinput

Procedure
Open a terminal window (CTRL+ALT+t) and install libinput with its tools on your system:

 sudo apt-get install xserver-xorg-input-libinput libinput-tools
Create a startup config file in /usr/share/X11/xorg.conf.d/ with the following command:

 sudo nano /usr/share/X11/xorg.conf.d/95-mouse-accel.conf
Paste the configuration to force the libinput "flat" profile on start-up:

 Section "InputClass"
            Identifier "My Mouse"
            Driver "libinput"
            MatchIsPointer "yes"
            Option "AccelProfile" "flat"
 EndSection
Press CTRL+o to save the file, then CTRL+x to exit nano.

Restart your machine. Alternatively, you can just restart your X server:

In Ubuntu:

    sudo service lightdm restart
In Linux Mint:

    sudo service mdm restart
Now you should have no mouse acceleration at all. Yay!