It seems — having had to deal with a quite a number in the last month or so — that most "answers" to old questions with an answer which is already accepted are in fact anything but an answer.
The most recent is:
nice answer..it cleared my confusion....
Generally these are automatically flagged by the system, which brings them to moderators' attention. Or, if such answers are sufficiently grammatical to bypass the automatic filter, members of the community flag them manually. Of course they are deleted as Not an answer, but this makes additional work for moderators, if not other members of the community.
Once a question has had such an answer, it's bumped on the site front page and tends to attract other answers which it seems are generally, although thankfully not always, of a similar quality.
Should there be a banner above the "Post your Answer" button advising users that there is already an accepted answer?
Such a banner would not necessarily prevent anyone from posting an answer, and it might be limited to users under a certain rep level, or to questions which are of a certain age.
It could be useful to have the system automatically protect questions when an answer is accepted (that is, prevent users with less than 10 rep from posting new answers), but this is probably unnecessarily restrictive; everyone has to start somewhere — even if starting on an old, accepted question is probably not the best place.
This Meta question asks only whether we should be reminding users [some users] that they should be posting something useful.