There is a method `onCreateThumbnail()`[1] that is supposed to allow you to do this, but it doesn't appear to work (reported bugs in 4.0-4.2). The best you can get at the moment is to blank out the thumbnail[2] (for security reasons) or opt out of being shown in the list [3].
As an aside, is there anything similar to this for android? I've tried a fair bit of googling but can't find anything or the right search term.