The answer to both your queries is that they cannot do anything. You must keep a copy of your resignation letter with you or send them your resignation by email and keep a copy. In case they refuse to accept your resignation and stop you from leaving the country, you can take the assistance of Human Rights Committee followed by lodging a complaint at the Labour Office. They will surely help you out.
Having said that, there are QLers who will post telling you that "employers are like your father and you should treat them nicely. Take them out to dinner and see the results. Everything will be fine ...Joys" etc.
It takes all sorts of people to make this world!