Some person is downvoting my answer. This answer is the accepted answer, but they cast downvotes on my answer. How to deal with such kind of users? How to complain to the admin of Stack Overflow?
The appropriate response, as with any down-vote, is to edit your post. If you honestly believe that you've done everything in your power to edit it and it's the prime example of what a post can be (and spoiler alert, I'm pretty sure I've never seen such a post), then it's time to simply accept the down-votes and move on.
I promise you, there's no way that either "admins" or mods will want to hear a complaint about down-votes. As it was mentioned in comments, multiple down-votes mean multiple users took issue with your post, and that typically means there are issues with it. Complaining will only attract more, unnecessary attention, and they certainly won't delete your post for you unless it's spam, should-be-a-comment, or has some other flaw like that. Before you object to my claim that they simply won't care, I'll back that up with a "there's nothing they can do about it." While mods will, yes, have the ability to delete a post, they cannot affect votes in a direct way.
Long story short, it is not "such kind of users'" fault, that's not how SE works. Focus on improving your post as much as you can, even if that means actually reading carefully through anyone who's commented to show concerns.
If any of your posts receive downvotes, it is an indicator that it may require improvement. Revise the post again, edit it to include more information, as well as try to incorporate any feedback which you receive in comments from the other members of the community. If you feel you have fixed the post to the best of your abilities, and there is nothing more to be improved, then move on. Everyone receives a few downvotes once in a while.
Accepted answer indicates that it was the most helpful answer for the OP, it is not an indicator of the quality of the answer. Accepted answers can be downvoted by the community, it is valid and there is nothing you can do in regard to this. OP decides the answer that helped them by accepting it, but the community voting indicates which is the best answer in terms of the quality. Many accepted answers have negative scores, you may find this interesting What is the most downvoted *accepted* answer in Stack Exchange history?
However, if a single user downvotes many of your posts, it is automatically detected and reversed as serial downvoting by the script that runs on all sites to detect voting frauds.