I'm sorry to hear your thinking of leaving Freedom and that you don't think its good for you. You can indeed join Google Adsense after you have left Freedom, in fact it would automatically revert back to this if you had an account prior to joining Freedom.
So to answer the question in the title in order to leave Freedom you need to go to your YouTube status and features page and then click the 'remove access' button. Once you have done that you just need to wait 30 days and then you will be officially unlinked from Freedom