As Zanna says, the appropriate action when you have raised a flag that you have then realized is wrong or unhelpful is to retract the flag.
Most (but not all) types of flags can be retracted. Flags on questions and answers, including the custom "in need of moderator intervention" flag where you type in a description of what is wrong, can be retracted. If the flag hasn't been handled yet, then you can retract it by clicking the flag link on the post, as you did to flag it originally. The option to retract the flag will be presented to you there. See "Is there any way to remove flags?" on this page for more details.
If you cannot retract the flag, it was probably already handled. To see what happened with any flag you have raised, you can view your flagging history. On the Activity tab of your profile, under ɪᴍᴘᴀᴄᴛ, there is a link that says how many flags you have raised that have been marked helpful. You can click that to go to a page with full details about all the flags you have ever raised.
This is only a link for you and for moderators--other users, like me, can only ever see a number there, but for us it's not a link. The link you see there should be https://askubuntu.com/users/flag-summary/297529, and you should be able to use that link from here, too. Because I am neither you nor a moderator, I cannot actually follow that link successfully--for me, it goes to a "Page Not Found" error page. In this way, your privacy is protected. In particular, non-moderators like me cannot read the text of your custom flags, nor can we see how your flags were specifically handled. (That's why I can only give instructions to help you find this information.)
Responding to Unhelpful Answers
If a moderator has already declined your flag, then there is nothing more you need to do to prevent the flag from being actioned. But you seem also to want to indicate that the answer, in spite of having automatically received a portion of your bounty, was not helpful.
If you can critique the answer in a way you think might be helpful to others, or that would help the author improve the answer, then you should probably post a comment on the answer. These official guidelines offer advice as to when comments should be used. You have enough
fake internet points reputation on Ask Ubuntu that you can comment on any post. However, even if you had less than 50 reputation, the system would still allow you to comment on your own questions and answers and on other people's answers to your questions.
If you've already commented then there is probably nothing more for you to do about that answer. In particular, there is no way to remove the bounty that has been awarded. When a user places a bounty that expires, the system awards half the bounty to the answer with the highest score (as determined by upvoting and downvoting), provided that there is an answer whose score is at least 2. See this page for details. That answer is shown as having received a bounty, but it is not shown as having received a bounty from you. Instead, the hover text shows the bounty as being from the Community bot.
So the situation of offering a bounty that expires and is then automatically partially awarded to an answer you don't like is somewhat similar to the case of another user offering a bounty on your question (you don't have to be the original poster of a question to offer a bounty on it) and then awarding it to a post that didn't help you. People should not, and hopefully will not, assume that the post helped you, just because someone (here, the Community bot) gave it a bounty.
If the answer was not helpful to you, then you can avoid giving the false impression that it did help by not accepting the answer. If you believe the answer is low in quality, wrong, and/or not helpful to anybody, then you could downvote the answer. Aside from that, there's not much you can do to undo the effect of the bounty, just as you cannot undo the effect of other users' votes on the post.
Placing Another Bounty
However, you have the option to offer a second bounty on your question. The second bounty will have to be for at least twice the value of the first one, so it's possible that you don't currently have enough reputation to offer it. (See this answer and that one.)
If editing your question would help clarify what you need, then you may want to do that. Whether or not you end up posting a second bounty, that might help.
As for "edit[ing] the offending answer to make it less harmful," please note that an answer being unhelpful to you is not really a good reason to edit it. As the tour says, Ask Ubuntu is intended to be "a library of detailed answers to every question about Ubuntu." However, if you have a revision to the answer in mind that you think would improve it and that is consistent with the guidelines about when to edit other users' posts, then you can go ahead and edit the post. Until you have 2000 reputation, your edits on other user's posts (except "community wiki" posts) will be reviewed by others before being applied.
Sometimes a post will be unhelpful to you but an edit is still not justified. In particular, edits that change the meaning of a post should generally not be applied to other users' posts. If an answer contains sections addressing aspects of a problem that don't apply to you, then even though you are the original poster, you should generally not remove those by editing, since they might help someone else with a similar problem. Most of the time, you should not apply any edit to another user's post that you expect the post's author to disagree with. In the rare cases when such an edit is justified, it's almost always best to solicit consensus from others before applying it.
Soliciting More Specific Advice on Meta
I've focused on general site usage in this meta answer. Just doing that, this answer is already plenty long, so I feel like it's reasonable for me to limit this answer to general advice. However, perhaps you are looking for advice that is specific to your situation. If so, then I recommend you edit this meta question to include links to your question and its answer, and possibly also to explain why the answer that received a partial bounty is unhelpful and why you think it might be reasonable to edit it.
It's possible to go to your profile and find the question and answer. So users probably can already talk about it here, even if you don't edit this meta question. But I have avoided doing this, since I'm not sure you're really looking for that sort of specific advice. If you are looking for it then I do recommend including links in your meta question.