Is it possible to turn off mouse middle button light?

Solution 1:

There's a general way:

1. Download AutoHotkey, a great freeware script language.
2. Install it, together with its editor (SciTe4AutoHotKey).
3. Click File > New.

4. Use the following code:

   #InstallMouseHook
   MButton::
   

5. File > Save, select a name (e.g. mouse.ahk)

6. Put a shortcut for this file in your Startup folder.
7. Restart the computer.

Solution 2:

A little hardware hacking could help.

I don't know details about your mouse but a little screwdriver and a little (more) patience could let you find out what produces the light (probably @Matt Jenkins is right) and if you can turn it off or just use a little black sello tape to hide it.

Have fun :)