As soon as you are listed as a participant, you can sign yourself out. To do that, simply click on the gray button "Sign me out".
Where to find the "Sign me out" button:
Event editors and Confluence administrators can always add/remove event participants. Even if those functions are disabled for normal users.
Want to know how to sign others out? Click here!
Link to this page: https://seibert.biz/events-signout