Turning off keyboard popup
DISCLAIMER:  I have never tried this - though it should not hurt your system.

You could remove /System/Library/LoginPlugins/BezelServices.loginPlugin... but removing this package would take all the bezel style interface items with it.  (volume up/down, brightness, remote paring, etc.)

Removing this package could have undesired effects, beyond just removing the bezels, but I can't think of any specifically...   It should work...  However, keep a copy of it, make note of the permissions, and put it back if it dorks other stuff up.

Also, expect later updates of OS X to replace this package...

Some may suggest viewing the contents of the package and removing just the bezels you don't want from /System/Library/LoginPlugins/BezelServices.loginPlugin/Contents/Resources/BezelUI/HiDPI but I notice there is a _CodeSignature directory here: /System/Library/LoginPlugins/BezelServices.loginPlugin/Contents/_CodeSignature and that may invalidate the entire package.

After making whatever changes you want to the BezelServices.loginPlugin you should probably restart your computer, or at the very least log out and back in - before you will see the desired effect.

Sorry for not providing a clearer answer.