Here is a solution that seems to work on most devices

1. Load up Netflix and go to the main screen
2. On your remote, use the directional arrow keys to enter this combination: Up Up, Down Down, Left Right, Left Right, Up Up, Up Up, Up
3. (That's 2 Ups, 2 Downs, Left - Right 2 times, then 5 Ups)
4. You should get the following (or similar) dialog popping up, with an option to Deactivate this Netflix device
5. Go ahead and do so
6. Then just re-enter your Netflix login and password after re-opening the app

Solution courtesy of GEEK DROP