Be logged in; use the deletion link.
UPDATE: the direct link currently no longer works, yet is still available on their website. This might be temporarily. We’ve contacted Alltrails about this issue.
URLS:
http://alltrails.com/
https://legacy.alltrails.com/member/delete
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.