First the question was bumped because someone added an answer. This is, and always will be, intended behavior (non-bumping "minor" edits have been refused multiple times, and with good reason). Of course, this is the same for questions with an accepted answer as well - they will be bumped if new answers are added (again, intentional behavior).
We could protect (stop low-rep users answering) or lock (stop anyone doing anything) the question to prevent a few these, but why? If it's a valid new answer then there's no harm it being added (if it's not we need the question bumping so we can see and remove it).
As for the acceptance of an answer:
Currently no one except the original asker can accept an answer.
Even in the case of the original asker being NA, as in this example.
Providing a moderator or community way to override this has been discussed many times (see links at end) and it is very unlikely anything of this nature will ever be implemented.
As such I'm afraid that flagging them for us (or anyone else) is pointless.
Moderators accepting answers on user's behalf after a certain time period
Allow users to mark "accepted" answer on someone else's question?
Plus loads of inbound duplicate links (check the "linked" on the right of these two).