We give users the option of setting up their group so that anyone can join it without the need to be invited. This makes life easy in some ways but it does expose you to the risk that someone you don't know will join your group and add steps to your group walk. Occasionally this does happen and once they have added steps to your group walk, or even completed it, there is nothing we can do to fix that for you.
We recommend you either make your group an invite-only group or make it a private group which means that it will be hidden from view and only invited users or those with the URL will be able to view/join.