It has been a long waited feature since ability to invite external users to join the organization but this is now there: you can now leave by yourself any organization you have been invited to join.
To do so just login to your Azure AD Access Panel Application (https://account.activedirectory.windowsazure.com) and open the Gear menu available at the right side of the Organizations section from the menu, or reach out the URL https://account.activedirectory.windowsazure.com/r#/profile/organizations
Then scroll down the page as the top displays devices associated with your account (aka Azure AD Joined) and click to the Sign in to leave the organization which will then log you on the inviting organization; you can review to what you have access and confirm your decision to leave or not
Go back again to the Organization section (see above), now the link Sign in to leave becomes Leave organization
Once you click on the Leave organization link you will be requested to confirm your choice
After hitting the Leave button to confirm you will get confirmation you have left the inviting organization and within the next few minute you will also a confirmation email