How to remove the possibility of giving himself the best answer?

Question

Each user can give himself the best answer. This is nonsense!
The best answer should give only the author questions!

Answer 1

I'm not sure it's a bad idea to let people select their own answer as the best. What if they did answer their own question? Q2A ensures they still don't get the points that another user would get for being selected as the best.

Still, if you want to do this, you can change following line in qa-page-question.php:

if ($question['editable'] && !$answer['hidden']) {

... to ...

if ($question['editable'] && (!$answer['hidden']) && (!$answer['isbyuser'])) {