I asked B&N support about the sample books when the STR first came out. I was told that there is no way to stop the sample books from being put into your library or sent to your Nook. Your only option is to delete them from your library on the web site then refresh your library on the Nook.
There is a setting on the Nook itself in the preferences to require your password to make a purchase with the Nook. This is your best bet for preventing accidental purchases.