Unable to select best answer

Hello Linguo_bastardo;

If you create a Question topic - you have the option to choose a 'Best Answer'
If you create a Discussion or Poll type topics those will not have a 'Best Answer' option.

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Your topic here was created as a Discussion topic.
 
No real method for the thread starter to do that.
You can just walk away to let it age gracefully and fade away.

If there's an important reason to lock a thread you can ask for help from the Moderator crew. That Report button will get their attention.
 
I think there is a "delete whole thread" function, when you edit the first post.

But there is really no need for you to delete the thread. They will die off with time and closed.