Took me few weeks to find out that this one-liner does wonder:
echo manual >> /etc/init/mysql.override
(the line above is to disable mysql, obviously. And must be done as root)
The answer is on first hit (as of this post’s writing) of googling “ubuntu disable service” but you need to scroll down a bit and ignore shitload of crappy, outdated explanations to find that small gem.
Unfortunately doesn’t apply to previous LTS. Or does it?