I recently gained review tools on SO, and I noticed that this answer just got added to a year-old question with a great, upvoted and accepted answer. The answer that has been added could answer the question (I'm not sure), but it is obviously far inferior to the accepted one. What does one do about this? Is it still valid- or should I flag it something?
Downvote the answer, and upvote the great, accepted answer. Why would any more be necessary? I don't see what is special about this compared to any other time a bad answer is posted.
If you feel really strongly, you can always leave a comment on the answer and ask the user to remove it. (Or, if you have at least 20k reputation, cast a vote of your own to delete it.)
If it's happening repeatedly on the same question (e.g., it appears to be a popular target for "spammy" answers), flag the question and ask for it to be protected, or protect it yourself if you have 15k rep.