After you've signed up for an activity or event in GroupValet, it's easy to cancel, that is if the group or community manager allows cancelations through GroupValet.
When you log into GroupValet and open that event, you'll see a green thumbs-up showing you're signed up. If the manager allows cancellations, you'll also see a red Cancel button; click it to cancel your signup. If there's no Cancel button, the manager has disabled cancellations in GroupValet, and you'll need to contact the group manager directly.