Short question, many solutions
The easy way
Using the Terminal, unregister all the links from you file types to iTunes
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -u /Applications/iTunes.app
If you still have trouble and iTunes opens automatically, check out RCDefaultApp. Old, but still working (right click->open...)
Install, go to system preferences, disable all the "media" links to iTunes
"Quick and dirty" (Teminal)
sudo chown -R $USER /Applications/iTunes.app
chmod ugo-x /Applications/iTunes.app/Contents/MacOS
This will prevent iTunes from starting EVER.
To start iTunes manually, do
chmod ugo+x /Applications/iTunes.app/Contents/MacOS/iTunes; open /Applications/iTunes.app; chmod ugo-x /Applications/iTunes.app/Contents/MacOS/iTunes
The way for the brave
Use duti, http://duti.org/documentation.html. Install using "brew install duti", make sure to have homebrew installed first (http://brew.sh).
Remove all links to iTunes.
Make sure to read Change file association in terminal? to get all current file types.