Not possible by default
The keyboard shortcuts only allow you to change selection once there is one. Reloading is the only way to stop having a selected post.
Possible with some custom code
To remove the selection it is enough to remove the "keyboard-selected" class. The following jQuery code will achieve that:
$(".keyboard-selected").removeClass("keyboard-selected")
This can be converted to a bookmarklet:
javascript:(function()%7B%24(%22.keyboard-selected%22).removeClass(%22keyboard-selected%22)%7D)()
To add it to your browser see How to Create Bookmarklets
This could also be incorporated into a userscript or a browser extension (for example, it could be executed when pressing a key).