If you think an answer is not helpful or is harmful, give it a down vote and/or post a comment under the the answer with your concerns. Possibly the answer will be edited to address your concerns.
Next if you know or can research a better answer, post your own answer. The Populist Badge exists to recognize posters of these answers.
Highest scoring answer that outscored an accepted answer with score of more than 10 by more than 2x. This badge can be awarded multiple times.
It is not unusual for a new user to post a question, get a quick answer, and except that answer. Maybe the OP never comes back. So the answer stays accepted. Or maybe this really is the the answer the user needed, accepting is a privilege granted only to the question asker. This is occurs across all SE sites and is also addressed here (I have also seen this issue better addressed in other places but only finding this one currently)
The nature of SE sites allows for both good and bad answers to be posted and accepted. Community voting brings the best answers to the top.
If the answer is really bad, when it has a score of -1 or lower the community can vote to delete it