Just out of curiosity, are these just random book samples that are downloading to your nook all the time or are these the samples that B&N adds to your account when you first register your device? So far, the only unwanted samples I've had were the ones BN sends to to the device when it's registered. I've had to reregister the device twice (once because the "What to what to read next" area only had one entry (some erotica book) and wouldn't update and once because I had to reset my account password) and BN automatically added a couple of samples to my account each time I reregistered.
Reregistering the device is a pain, and you will have a couple of samples to delete afterwards, but it might be worth a try if you are continually getting unwanted samples. Could be some setting on your device didn't reset and the BN website keeps thinking that your device is new. I hope BN isn't going to start sending samples out all of the time - the "what to read next" area is as much advertising I want to see.