Don't enter a discussion with them. As andyn said, send them a letter; if they argue, don't bother arguing back, just ignore them to the best of your ability. I

Don't enter a discussion with them. As andyn said, send them a letter; if they argue, don't bother arguing back, just ignore them to the best of your ability. If you have no binding contract, you are under no obligation to continue working for them, nor are you obliged to explain why you refuse to work for them.