How can I force a dictionary in python to reject updates of existing keys?

Just check your dict before you add the item

if 'k' not in mydict:
    mydict.update(myitem)

You can always create your own dictionary

class UniqueDict(dict):
    def __setitem__(self, key, value):
        if key not in self:
            dict.__setitem__(self, key, value)
        else:
            raise KeyError("Key already exists")