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, for that event it should show a green banner saying you're signed up. If you also see a red Thumbs Down button, then clicking that will cancel you. If you don't see a red Thumbs Down, that means the manager has disabled cancelations in GroupValet. You will need to contact the group manager directly in that case.