Last times i used iTunes store in later 2009, after that, this section in iTunes start annoying me. And discovered this way, to disable it:

open Terminal and write this:

defaults write com.apple.iTunes disableMusicStore -bool true

This will disable iTunes Store section, but if you wish to revert, just do:

defaults write com.apple.iTunes disableMusicStore -bool false

P.S. Also this is another way to remove annoying arrow to iTunes Store near each song.