According to their website, it is very easy, there is no obligation and nothing to pay. However, some users have reported problems - if you put 'problems cancelling netflix' in a search engine you will see some comments like these. Most complaints are to do with Netfix failing to activate the cancellation and the user having a long struggle to get it recognised.
It's only fair to point out, though, that people do leave Netfix and that there are problems for some consumers with cancelling most things (it's incredibly difficult to cancel a store card, for instance, which is why I will never have one now). The fact is that most companies don't like you leaving and won't necessarily make it easy for you, so before signing up to anything you should read all the terms and conditions very carefully, and check user reviews using search terms like the ones mentioned above.