Install packages:
sudo apt-get install xbindkeys xsel xdotool
Create xbindkey configuration file:
# ~/.xbindkeysrc
"echo -n | xsel -n -i; pkill xbindkeys; xdotool click 2; xbindkeys"
b:2 + Release
Set xbindkeys to load on startup:
# ~/.bashrc
xbindkeys -p
Log out, log back in, and you should be all set. If you are impatient, simply run xbindkeys -p
A much better way (if you are using something gnome-based) is to just use
gnome-tweaks
to disable middle click paste directly