De-activating the account that you no longer want to use on the Netflix website should have done the trick. When you disable or deactivate a device, it's no longer recognize when it connects to Netflix - it would then typically give you the opportunity to enter new code with the new account that you want to use.
But it seems that this isn't the case in this situation, and fortunately there is one remedy I can suggest.
With the unit powered on, press and hold the EXIT button on the remote control for 15 seconds. You should be prompted to "RESET?" Choose YES and [ENTER].
Now, log into your television and connect your Internet. By launching the Netflix App, you should be prompted to enter a code at www.netflix.com/activate or www.netflix.com/samsung
The television will now be referred back to its original factory settings, meaning that you'll have to go through the entire set up as well. If your television was calibrated, you may lose your picture and audio settings.
I recently bought a new Samsung TV (UN40D6000SF I beleive). Well I set up a netflix account with my Mom's account while she was here. Well long story short now I want to set it up with my own account. Already have my new account set up online and deactivated the TV on the other account and all but can't seem to log off my Mom's account on Netflix app on Samsung to my own, searched the whole internet and tried mostly all buttons on my remote but nothing. Can anyone help on this?