It is way too high for me to be able to aim, I have pointer speed turned down to minimum in mouse settings, and joystick + mouse sensitivity turned down to minimum in-game.
How can I decrease it further and turn mouse acceleration off?
Have you tried adjusting your mouse in Windows? Or if you really want you can buy mice with adjustable speed on the side quite cheap now. I think they call it dpi on the fly.
My suggestion is to pick up a Logitech gaming mouse when you can get one on sale. They have adjustable DPI and control software.
Here is one that has good ratings
http://www.amazon.com/Logitech-G300s-Optical-Gaming-910-004360/dp/B00RH6R7C4/ref=sr_1_1?ie=UTF8&qid=1464058163&sr=8-1&keywords=logitech+gaming+mouse