I had a similar problem before. As quartermaster (but not owner), you can only unassign yourself as carrier. You need an owner to unassign you as quartermaster.
In the Items inventory view (not "Items New"), I used the output feature to display all my assigned items. The output feature tells you who the owner of the items are. You can contact the owners to ask them to remove you as quartermaster.
Although, with over 800 assigned items... that sounds like a headache. Sorry I don't have a better suggestion.