How can I stop iTunes downloading artwork?

After downloading iTunes 11 I noticed that iTunes went ahead and downloaded a bunch of missing artwork for some of my albums. Call me picky but I'd chosen to leave these items without artwork, and so I deleted it. No matter how many times I do this, and by what method, iTunes keeps adding artwork to these albums.

I've made sure that 'Automatically download album artwork' is unticked in Preferences > Store.

I've even unticked 'Use custom colours for open albums...' in Preferences > General in case that was overriding my choice of no album artwork.

I've tried both manually deleting the artwork using 'Get Info' from the context menu, and 'Clear Downloaded Artwork' from the same menu, without any luck.

A little more unnecessary detail: these are largely albums of various songs by an artist that I don't want an 'album' image applied to since they are not albums per se. For example, this collection of Arcade Fire b-sides which iTunes keeps adding the Hunger Games album artwork to since that's where one of the tracks originated:

Any and all help appreciated.